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THE STUDY OF

PLANT COMMUNITIES

An Introduction to Plant Ecology

HENRY J. OOSTING

Associate Professor of Botany
Duke University

1948
W. H. FREEMAN AND COMPANY

San Francisco, California

All rights to reproduce this book in whole or in
part are reserved, with the exception of the right
to use short quotations for review of the book.

PRINTED IN THE UNITED STATES OF AMERICA
TYPOGRAPHY BY MACKENZIE & HARRIS, INC., SAN FRANCISCO

TO
MY STUDENTS

from whom

I have learned much more

than they realize

Preface

This book grew out of several successive reorganizations of an
introductory course in plant ecology. Since it is intended as an
introduction to plant ecology, effort has been made to make it as
stimulating as possible while presenting basic information. From
experience we know that this ideal is best achieved through study
of plant communities with emphasis on field work. The plant
community, therefore, is made the basis of this book.

The plan, in brief, proceeds from a consideration of the nature
and variation of communities to methods of distinguishing and
describing them. This is followed by a discussion of the factors
which limit, maintain, and modify communities both locally and
regionally. Thus the interrelationships between organisms and
environment are emphasized and a foundation is laid for presenta-
tion of the concepts of succession and climax. Then the climax
regions of North America become a logical consideration since
they are illustrative of all that comes before. To answer the ques-
tions which must arise regarding the permanence of climax, a sec-
tion is devoted to past climaxes and their study and reconstruction.
Finally, the potentialities of the ecological point of view in prac-
tical considerations are emphasized by a survey of its possible and
desirable applications in range management, agriculture, conserva-
tion, landscaping, forestry, and even human relations.

The intent has been to write a textbook with a wide usefulness.
It was assumed that, in some instances, the text material might
serve as the complete subject-matter of a course. To this end, the
presentation aims at a fairly broad but solid foundation for eco-
logical thinking and appreciation. At the same time there is no
attempt at completeness, either in subject matter or bibliography,
such as might be expected in a reference volume. Although con-
troversial issues are not deliberately obscured, they are not em-

PREFACE

phasized. The assumption has been that a beginning student should
acquire a working knowledge and appreciation of the field before
he is introduced to matters that might confuse him.

A reasonable background of botanical and scientific experience
is assumed so that, in general, college juniors and seniors might be
expected to have the greatest appreciation of a course of this kind.
A reasonable knowledge of plant physiology is expected, at least
enough for comprehension of ordinary physiological processes.
Although a student without some taxonomic training could hardly
fully appreciate or enjoy an ecology course dealing with com-
munities, he could use this book if he had some knowledge of
plants. Both common and scientific names have been given reg-
ularly or at least the first time a species is mentioned. The plants
which are named are almost without exception rather generally
known species of long standing. It is not considered necessary,
therefore, to include authorities with scientific names since they
may invariably be found in standard manuals.

Suggestions for collateral reading may be found in the selected
general references at the ends of chapters. Cited references are in-
dicated in the text by number only and are listed in the bibliog-
raphy at the end of the book. Citations are made where it seemed
desirable to indicate the authority for or give credit for state-
ments used in the text. Again, for those who may wish to go to
original sources, references to survey and review papers are in-
cluded. The bibliographies of these references are usually so ex-
tensive that the advanced student who uses them may quickly
accumulate all the source material he needs.

Those who contributed directly or indirectly to the develop-
ment of this book are too numerous to mention specifically, but I
am deeply aware of my debt to former instructors, my colleagues,
and my students. Many have given invaluable aid in the actual
preparation of the book. A very special acknowledgment of as-
sistance is due Miss Ruby Williams who, through a careful reading
of the manuscript, did much to improve the mechanics of organ-
ization and to clarify and simplify the presentation.

The use of the book in mimeographed form provided a test of
its value under a variety of conditions in different sections of the
country. It was used in classes by Dr. W D. Billings at the Uni-

8 THE STUDY OF PLANT COMMUNITIES

versity of Nevada, Dr. M. F. Buell at Rutgers University, Dr. R. B.
Livingston at the University of Missouri, and by Dr. J. F. Reed at
the University of Wyoming, as well as at Duke University. The
comments and suggestions derived from both students and instruc-
tors led to revisions and additions which are invaluable, particular-
ly in their contribution to wider utility. It is truly with deep
appreciation that the cooperation and assistance from all these
sources is acknowledged.

Finally, although credit lines indicate the sources of illustrations,
it is a real pleasure to acknowledge the courtesies and helpfulness
of the numerous individuals and organizations involved. The ex-
cellent material they made available, sometimes with considerable
trouble to themselves, often made it necessary to choose from sev-
eral possibilities for a single illustration. It is regretted that not all
the pictures could be used. The line-drawings were done by
George A. Thompson and Robert Zahner whose assistance is
gratefully acknowledged.

HENRY J. OOSTING

Durham, North Carolina
February, 1948.

Table of Contents

PART 1-INTRODUCTION Page

Chapter I. The Subject Matter of Ecology . ... 11

PART 2-THE PLANT COMMUNITY

CHAPTER II. Nature of the Community 21

CHAPTER III. Yegetational Analysis :
Quantitative Methods % 33

CHAPTER IV. Yegetational Analysis :
Phytosociological Objectives 55

PART 3-FACTORS CONTROLLING THE

COMMUNITY : THE ENVIRONMENT

CHAPTER V. Climatic Factors : The Air 75

CHAPTER VI. Climatic Factors : Radiant Energy.
Temperature and Light 11,5

CHAPTER VII. Physiographic Factors 144

Chapter VIII. Biological Factors 188

PART 4-COMMUNITY DYNAMICS

Chapter IX. Plant Succession 211

CHAPTER X. The Distribution of Climax Communities :
Present Distribution of Climaxes 234

Chapter XI. The Distribution of Climax Communities :
Shifts of Climaxes with Time 3 00

PART 5-PRACTICAL CONSIDERATIONS

Chapter XII. Applied Ecology 315

References Cited 3^7

Index ?71

63307

Part 1 • Introduction

CHAPTER I

THE SUBJECT MATTER OF ECOLOGY

"What is Ecology and What Good Is It?"250- the title of an
address made before the Ecological Society of America some
years ago, is a compact, perhaps oversimplified, statement of the
questions this textbook aims to answer. Its intention is to present
an adequate introduction to the various phases of the subject, to
show its position in relation to other sciences, and to indicate the
possibilities and advantages of applying the methods and point of
view of ecology in solving biological problems.

THE TERM AND BASIC CONCEPTS
The term, ecology, carries a more familiar ring than it did a
relatively few years ago. Although it was used commonly in many
fields of science, it did not, until recently, appear elsewhere. Now,
it is occasionally seen in magazines and sometimes even in news-
papers. This is partly the outgrowth of a gradual maturing of the
science and partly the result of a growing appreciation of its mean-
ing and potentialities.

Although the subject matter of ecology is as old as that of any
other science and although much of it has long been a part of sci-
entific knowledge, ecology as a field of science is relatively new.
The name first appeared, in 1869, as "oecology;'112 but the great-
est advancement has come during the past fifty years, following
the impetus supplied by the writing and thinking of a few men in
the late 1890's. The term "ecology" is particularly appropriate. Its
Greek root, oikos, means home and thus indicates a dwelling place;
this, of course, implies that organisms are present and that certain
conditions link the two. Ecology is, therefore, the study of organ-
isms, their environment, and all the interrelationships between the
two. It is commonly defined as the study of organisms in relation
to their environment.

12 THE STUDY OF PLANT COMMUNITIES • Chapter I

ALL LIFE BOUND TO ENVIRONMENT

An organism without environment is inconceivable,122 for living
things have certain requirements that must be satisfied by their sur-
roundings if life is to continue. Their physiological processes,
which, to sustain life, must all continue at rates above definite min-
ima, are largely controlled by environmental conditions or sub-
stances. Most of the processes use water or require its presence;
food manufacture is dependent upon carbon dioxide and light con-
ditions; the universal process of respiration requires oxygen; and
all processes are limited by, or vary with, temperature.

Since organisms must grow and reproduce to survive, they re-
quire energy, which they derive from food by respiration. Food,
therefore, becomes a major consideration in explaining the activi-
ties of organisms. Green plants must be able to manufacture
enough food to grow and reproduce and still leave a surplus for
dependent organisms. Among the latter, there are usually several
dependent upon each other for food in a relationship called a food-
chain. For example, in aquatic environments the food-producing
algae are eaten by miscroscopic animals that may in turn be eaten
by larger animals upon which small fish feed. Small fish are often
eaten by larger fish, and many of these are eaten by man. Any
number of things may disrupt such a food-chain, but, under nor-
mal conditions, all the organisms are interrelated by their mutual
requirement of food, whose ultimate production is dependent
upon algal activity in the presence of light.

Regardless of the environment and the group of organisms
adapted to survival in it, similar food-chains and dependencies can
be found everywhere. Thus we see that the basic relationship
binding all organisms to each other and to the environment is, in-
variably, one traceable to energy needs and uses; and, because the
ultimate source of energy for both plants and animals is the sun,
all organisms are mutually related to each other and to their en-
vironment.

If groups of organisms live together successfully, their demands
and effects upon the energy cycle will not disrupt it. All the proc-
esses and activities taking place within the group will be in balance
with the available supply of energy. A major concern of ecology,
therefore, is to learn what that balance is and what controls it.

THE SUBJECT MATTER OF ECOLOGY 13

ENVIRONMENT A COMPLEX OF FACTORS

Environment includes everything that may affect an organism
in any way. It is, therefore, a complex of factors, which may be :
substances, such as soil and water; forces, such as wind and grav-
ity; conditions, such as temperature and light; or other organisms.
These factors may be studied or measured individually, but they
must always be considered in terms of their interacting effects
upon organisms and each other. The resulting complexity of en-
vironment and the array of subject matter encompassed suggest
the necessity for drawing upon the knowledge of all fields of sci-
ence for its understanding. Therein lie a complete justification of
and explanation for ecology. Its special function is to consider
such subject matter in terms of organisms. Any one field of science
is relatively restricted to its own subject matter, whereas ecology
brings together the knowledge of various sciences with the object
of interpreting the responses of organisms to their environment.

Since all plants and animals, including man, are organisms, and
since environment can at times include almost anything in the uni-
verse, the subject matter of ecology is almost unlimited. As a re-
sult, it is dependent upon the specialized fields of science for much
of the knowledge it uses. It requires an understanding of the funda-
mentals of other sciences, an alertness to changes and new discov-
eries in various fields, and a constant consideration of the possibili-
ties of using such information for interpreting or explaining the
peculiarities, responses, and nature of organisms under the con-
ditions in which they live.

SCOPE OF THE FIELD OF ECOLOGY

Since the subject is concerned with organisms, it must include
both plants and animals. Such a broad biological basis presupposes
a solid foundation in both botany and zoology, and, if man is to be
considered, an additional need for understanding of sociological,
psvchological, and economic problems. Although the latter are
not ordinarily considered biological subjects, they may become
more so in the future. Sociologists are more and more concerned
with "human ecology'' and some phases of ecology have come to
be known as "plant sociology!'

It is, unfortunately, unusual to find students, teachers, or inves-

14 THE STUDY OF PLANT COMMUNITIES * Chapter I

tigators today with sufficient training or experience to deal ade-
quately with the entire field of biology. This explains why special-
ists usually concentrate on either plant ecology or animal ecology,
and why textbooks emphasize either plants or animals, even though
all organisms should be considered. In an introduction to the sub-
ject, however, it is probably advantageous to restrict the subject
matter for effective discussion. We shall, therefore, be concerned
primarily with the ecology of plants, although their relationships
to animals will not be ignored. Furthermore, the major emphasis
will be upon natural groupings or communities of plants and the
reasons for finding them as we do.

BROAD TRAINING DESIRABLE

At first thought, the diversity of subject matter included in the
scope of ecological application is discouraging. It ranges through
all the sciences, but obviously one person can hardly become mas-
ter of all scientific knowledge. Specialists, however, working on
different phases of a problem, can contribute to its solution, pro-
vided they all have the same objectives and points of view. Most
ecologists are specialists in some phase of the subject, but the
ecological approach provides the necessary unity for holding their
interests together. A truly complete ecological training is impos-
sible; yet it is possible to acquire a broad enough training to appre-
ciate the importance of subject matter in fields with which one
may not be entirely familiar.

An appreciation of ecology necessitates certain fundamentals of
training for a background. The specialist then expands his knowl-
edge along lines of interest. A basic biological foundation is, of
course, a necessity, with taxonomy and physiology as absolute
prerequisites because of their constant use. Because ecological
problems frequently range through any of the biological fields
from morphology to pathology to genetics, the advantages of an
extensive preparation should be evident.

The desirability of a basic understanding of physics and chem-
istry need hardly be emphasized since both have their obvious
uses in the interpretation of environmental conditions as well as in
applications to physical and physiological problems. Some knowl-
edge of geology is very useful, and, for certain types of work, a

THE SUBJECT MATTER OF ECOLOGY 15

broad training in this field is a necessity. Soils are a constant con-
cern of the ecologist both as to their origin and development and
as to the paralleling vegetational characteristics as modified by
water, aeration, and nutrition. The frequent recurrence of prob-
lems related to climatology suggests its desirability, and the in-
creasing use of quantitative methods requires an appreciation of, if
not actual facility in, the use of statistical methods and experi-
mental design. Also, ecological problems frequently overlap those
of applied fields such as agriculture, forestry, and range manage-
ment. In addition to the terrestrial ecology with which we shall
primarily concern ourselves in this text, there are the special fields
of limnology, dealing with fresh-water environments, and marine
ecology and oceanography with all their particular problems.

These suggestions are indicative of the diversity of subject mat-
ter included in ecology. Specialization is a natural and desirable
result so long as it contributes to the ultimate goal of understand-
ing the interrelationships of organisms and environment and to
clarifying the natural laws under which the complex operates.

HISTORICAL DEVELOPMENT OF PLANT ECOLOGY

The origins of modern plant ecology are, of necessity, diverse.
Designation of the limits and ranges of species by Linnaeus and
other early systematic botanists led to the development of floris-
tic plant geography, which considers the origin and spread of
species. The next step was in the direction of explaining distribution
of species. Humboldt, a taxonomist who was a great traveler, was
impressed by the correlations with climate that he observed. As a
result, he developed his ideas so effectively at the beginning of
the nineteenth century126 that the influence of his thinking is
still apparent in the interpretations of climatic plant geography.
Schouw,214 one of Humboldt's students, was the first to attempt
the formulation of laws regarding the effectiveness of light, mois-
ture, and temperature in species distribution. Somewhat later
(1855), still another taxonomist, A. de Candolle, published studies
along this line but with major emphasis upon temperature as a
controlling factor. Attempts to correlate vegetational distribution
with single factors continued for several years and culminated in
Merriam's173 study of temperature zones for all of North America.

1 6 THE STUDY OF PLANT COMMUNITIES • Chapter I

The geographer's preoccupation with climatic causes for the
distribution of species was paralleled by another significant trend
of interest initiated by the writings of Grisebach in the nineteenth
century. He recognized groups of plants, or communities, as units
of study and described the vegetation of the earth on this basis.111
This was the first step in the direction of modern studies of plant
communities. Although further expanded by the publications of
Drude,94 the trend received its greatest impetus from the writings
of Warming, particularly his Oecology of Plants,266 originally
published in Danish in 1895. This publication marks the beginning
of modern ecology as it is concerned with communities and the
interrelationships of organisms and environment. Although Warm-
ing must be credited with recognizing the complexity of these re-
lationships, he tended to place too much stress on water as a con-
trolling factor. In 1898, Schimper published his monumental Plant
Geography upon a Physiological Basis, which was later (1903)
translated into English from the German. Its author followed the
general plan of presentation begun by Warming but contributed
substantially from his broad experience and travels. He came near-
er to the modern interpretation of causes of distribution of vege-
tation by emphasizing the complexity of environment and the
interraction of factors.

These, briefly, are the foundations of modern community studies
and the philosophy of modern ecology. From them stem studies
of the structure and classification of communities as emphasized
by continental European ecologists particularly, intensive studies
of habitat in the search for causes, and analysis and interpretation
of vegetational change as developed by American and English
workers. The history of modern ecology is so brief that the last
of these developments can hardly be treated historically. They are
the fundamentals of ecology today and, therefore, will be consid-
ered as part of the text material of this book.

APPROACHES TO THE SUBJECT

Considering the diversity of subject matter in ecology and the

variety of possible interests, it is not surprising that problems have

been studied in many different ways. Certain investigations must

be made in the laboratory and others in the field. Some ecologists

THE SUBJECT MATTER OF ECOLOGY 17

have focused all their attention upon single factors; others have at-
tempted to analyze the joint effect of several factors.

Autecology and Synecology.— Certain problems can best be
solved by working with individual organisms or species in the lab-
oratory or in the field. Others can be solved only when the group-
ings of organisms are investigated as they occur naturally. Similar-
ly, the environment may be analyzed one factor at a time or
considered in its entirety as a complex of several factors. Each
approach has its merits under conditions that should become ap-
parent later. The two are distinguished as autecology— from the
Greek root autos meaning self— which deals with individual or-
ganisms or factors, and synecology— from the Greek prefix syn
meaning together— applied to studies of groups of organisms or to
complexes of factors.

Autecology is not always distinguishable from some kinds of
physiology; in fact, there is probably no point in doing so. The
very nature of autecology brings about overlapping with other
fields. Autecology is, nevertheless, justifiable because of the con-
tributions it can make to synecology. The latter is clearly a field in
itself whose objectives make it distinct from all other fields of
science. This is a partial reason for giving major consideration to
synecology in this text and for bringing in autecology only when
it contributes to the understanding of discussions of community
problems.

Static and Dynamic Viewpoints.— Plant communities may be
studied as they are, without regard to what mav have preceded
them or to what their natural future may be. This leads to con-
sideration of the abundance and significance of the species making
up the community and permits detailed descriptions and precise
classification of communities according to one system or another.
It is typical of the work of several early continental Europeans,
who, as a result, developed systems of classifying and describing
communities and their structure. In America and England, the
view was early adopted that a community is a changing thing
whose origin, development, and probable future can be recon-
structed or predicted. These two approaches have come to repre-
sent what are known as the static and dynamic points of view in
community studies. The static approach is undoubtedly a product

THE STUDY OF PLANT COMMUNITIES " Chapter I

FIG 1. Communities of contrasting life form as illustrated by vegetation
on Roan Mountain in the southern Appalachians. (1) Deciduous forest of
beech and maple. (2) Portion of a grassy bald in which grasses and sedges

of the restricted areas of study in Europe where civilization has
long since destroyed or modified most natural communities. In the
same way, the vast areas of virgin forest and grassland in America,

THE SUBJECT MATTER OF ECOLOGY

i*V- '

predominate. (3) Portion of a shrub community made up largely of rhodo-
dendron (open coniferous forest in background). (4) Moss community in
which young conifers are becoming established.— Photos by D. M. Brown.

with opportunities to observe natural variation on a large scale and
under a variety of circumstances, must have contributed to de-

20 THE STUDY OF PLANT COMMUNITIES • Chapter I

velopment of the dynamic point of view. Undoubtedly each
method has its place and usefulness. In fact, each has profited from
the other, but, since the dynamic point of view has the broadest
usefulness in both pure and applied ecology, it will be emphasized
here.

BACKGROUND FOR COMMUNITY STUDY

Systems of description of vegetation that are based upon appear-
ance or general nature of the plants have been used with some suc-
cess, particularly by plant geographers. Such systems indicate size
and form of plants; whether they are evergreen or deciduous,
herbaceous or woody;210 position of buds in the dormant season,202
and various other characters classified under the general headings
of growth forms or life forms. This makes possible the visualization
and superficial comparison of otherwise unfamiliar vegetation and
likewise may serve to bring out certain characteristics of com-
munities that otherwise might not be apparent. Such systems are
either based upon previous detailed studies of the species, or they
may be a means of superficially characterizing vegetation of which
the taxonomy is still inadequately known. They can only supple-
ment studies based upon taxonomy since description of a commu-
nity, to be adequate for all purposes, must be based upon species.
The field ecologist must, therefore, have a thorough working
knowledge of taxonomy and, preferably, some experience with
the flora of the region of his studies

Just as the study of vegetation must remain more or less super-
ficial without a solid taxonomic foundation, so will interpretations
and explanations be limited by the amount of autecological infor-
mation available about the species and their environments. Physi-
ological-ecological investigations, in the field and under natural
conditions, constantly modify synecological conclusions that have
been made deductively, or they suggest new interpretations and
investigations. The quality of community studies, therefore, de-
pends upon certain fundamentals, which include a knowledge of
the individual species and their requirements and responses.

Part 2 • The Plant Community

CHAPTER II
NATURE OF THE COMMUNITY

Recognition of a plant community or distinguishing one com-
munity from another is probably simpler than recording the char-
acteristics by which the community is recognizable. To refer to a
stand of pine, a grassy field, or a lowland forest is, in a sense, rec-
ognizing communities, and most of us have done this from child-
hood. Such communities are the basic vegetational units of the
ecologist, and, therefore, their specific and general characters
should be stated to insure agreement as to concepts.

DEFINITION

A good working definition is as follows: A community is an
aggregation of living organisms having mutual relationships among
themselves and to their environment. This applies to the specific
example which one has in mind or which one is observing— that is,
the concrete community or stand. At the same time, it does not
exclude the possibility of visualizing an abstract community syn-
thesized from several or many concrete examples or stands. Thus
a particular stand of pine would be a concrete community and the
community in the abstract would include all the stands of that
species.

A stand need not be limited to trees. Any group of plants satis-
fying the definition of a community may be so termed— a mat of
lichens on a rock, covering only a few square inches, an algal mat
on a pond, or a forest of fairly homogeneous composition extend-
ing- over a thousand acres or more.

MUTUAL RELATIONSHIPS AMONG ORGANISMS

These include all the direct or indirect effects that organisms
have upon each other. Foremost among these is competition,

22 THE STUDY OF PLANT COMMUNITIES • Chapter II

FIG. 2. A stand of mixed conifers in Idaho— U. S. Forest Service.

which results whenever several organisms require the same things
in the same environment. The intensity of competition is deter-
mined by the amount by which the demands exceed the supply.
Competition may occur between individuals of the same species.
Because they are alike, their demands are identical, and, if the sup-
ply of water or nutrients or light is insufficient to satisfy the needs
of all, then some will be eliminated. This is particularly notice-
able in young, crowded forest stands but is equally true among
roadside wreeds or in a vegetable garden. All plants may survive
for a time in a stunted condition; then some individuals are gradu-
ally eliminated. Whether in the forest or in the garden, thinning
to reduce competition between species usually pays with more
lumber or better vegetables.

Stratification.— Usually there are several species involved in
competition within a stand. If plants of several species that start
simultaneously make the same demands upon the habitat, they may
survive in about equal numbers and occupy the same position in
the community. Those whose requirements differ will affect each

NATURE OF THE COMMUNITY

other less but will most certainly not be of equal importance in the
community. A tall-growing species outgrows a potentially short
one under the same conditions. If the latter then survives, it does
so because its light requirements are not great. Thus the one tends
to occupy a higher level than the other and to form an overstory.
In this way stratification may develop in a stand in which the
upper stratum of plants usually includes the controlling and char-
acteristic species for the community. These are termed the dom-
inant individuals. If they are removed for any reason, as by selec-
tive cutting or disease, dominance is usually assumed by other
species, and the character of the community is changed completely.
This is not true when lesser species in subordinate strata are re-
moved, for, with the dominants intact, the same type of commu-
nity can regenerate itself.

Stratification may likewise be seen among the shrubs and herbs
beneath the trees, since some may be tall and some low. The lowest

Fig. 3. A stand of moss (Hypnum crista-castrensis) on the forest floor in
northern Wisconsin. Although this species is a dependent within the forest
community, it forms a stand nevertheless— Photo by L. E. Anderson.

24 THE STUDY OF PLANT COMMUNITIES ■ Chapter II

FlG. 4. Very much overstocked stand of naturally seeded, eight-year-old,
loblolly pine. Although many individuals will die in the next few years and
thus produce natural thinning, the remaining trees will remain spindly and
growth will not be satisfactory. Artificial thinning to reduce competition is
apt to pay dividends in such stands.— Photo by C. F. Korstian.

Fig. 5. Young loblolly pine stand, which was overstocked (left) for best
growth. The stand was thinned experimentally soon after it was photo-
graphed. Same stand (right) onlv two years after thinning, shows marked
increase in size in the reduced number of trunks. The increase in rate of
growth will be apparent for a number of years.— Photo by C. E Korstian.

exposed stratum is made up of mosses, lichens, and sometimes
algae, which may form a mat or ground cover on the forest floor,

NATURE OF THE COMMUNITY

and a final stratum of fungi, bacteria, and algae in the upper layers
of the soil can also be recognized. The species making up these
lesser strata probably offer little direct competition to the trees
above them. Most of these plants have appeared, and are able to
survive, here because of conditions provided by the tree strata.

FlG. 6. Stratification in an oak-hickory forest community as seen in spring
when the subordinate tree stratum is especially marked by flowering of dog-
wood and redbud.— Photo by H. L. Blomquist.

Indirectly, however, they may offer serious competition to the
continued dominance of the trees because, if the trees are to main-
tain themselves in the community, they must be able to reproduce
themselves. If the seedlings of tree species cannot meet the compe-
tition of lesser species, whether it be in the herb or shrub stratum,
such trees must eventually disappear from the community. Thus
permanent or true dominance involves the ability to compete suc-
cessfully in all strata of the community. The effects of competition
are most apparent in the lesser strata, and undoubtedly competi-
tion is greatest between the seedlings of species of all strata since
all must start small and in the same restricted environment of the
forest floor.

Some ecologists maintain that each of these strata is itself a com-
munity (synusia), which should be considered as a distinct unit of
vegetation. Whether or not the strata are so recognized, they can-
not be neglected in any study of communities. Often an under-

26 THE STUDY OF PLANT COMMUNITIES * Chapter II

standing of the community as a whole is possible only after infor-
mation is complete on the individual strata.

Dependence.— Within any community some species, although
a part of the community, are at the same time dependent upon the
whole for their survival. To a great extent, these are inconspicuous
organisms, which, at first glance, might well be overlooked or
ignored. Most of the bryophytes and thallophytes, as well as a few
vascular plants, require the special conditions provided by larger
seed plants; shade and moisture are usually of greatest importance
to their survival. Such dependent organisms would soon disappear
if the dominant vegetation were removed.

Epiphytes grow on the trunks, the branches, and even on the
leaves of the larger plants. In subtropical and tropical forests they
may be conspicuous because of both size and abundance. In for-
ests of temperate zones they may be easily overlooked, for they
are usually mosses, liverworts, or lichens. These may be restricted
to certain communities, and sometimes individual species will grow
only on specific trees. Fungi, including bacteria, make up an im-
portant part of many communities, especially forests. Here they
may be parasitic and cause diseases that may at times become so
serious as to destroy a stand or even to eliminate a community.
Other saprophytic fungi, living in the soil or litter of the forest
floor, although dependent upon the community, likewise contrib-
ute to its perpetuation through their activities in decomposition of
organic matter. Still others, again often host specific, live in an as-
sociation with the roots of vascular plants in a relationship termed
mycorhiza (see Fig. 91).

Finally, animals, largely as dependents but also as influents, are
likewise a part of the biotic community. Large species such as deer,
which move about freely, are not necessarily associated with a
single community. However, many smaller, less widely ranging
species are definitely restricted to single communities, and even
some birds and flying insects may be constantly associated with
certain types of vegetation. Many beetles, borers, moths, etc. are
extremely destructive parasites, while other similar small animals
live on the remains of dead plants. The animals are apt to be re-
lated to the community through food requirements and, if present
in large numbers, may have extremely destructive effects.

NATURE OF THE COMMUNITY

MUTUAL RELATIONSHIPS TO ENVIRONMENT

Plants must be adapted to the environment in which they live if
they are to survive for long. Some can withstand heat, some cold;

28 THE STUDY OF PLANT COMMUNITIES ' Chapter II

some require a large continuous supply of moisture, others require
only a small amount which need be available only periodically.
Thus the climate of a region definitely controls the kinds of plants
that may grow there. The general vegetation type or growth

FIG. 8. Spanish "moss" (Tillandsia usneoides), an epiphytic flowering
plant, growing on live oak, North Carolina coast— Photo by H. L. Blomquist.

form, such as grassland, desert, or forest, is a product of the com-
plex of climatic factors effective in a region and can be used as a
generalized basis for evaluating the climate. For example, knowing
something of the growth forms able to survive under the extreme
conditions of moisture and temperature associated with a desert, a
repetition of these growth forms anywhere else in the world auto-
matically may be accepted as indicative of desert conditions. The
scrubby broad-leaved evergreens (chaparral) that cover much of
southern California are a product of the climatic conditions pe-

NATURE OF THE COMMUNITY

culiar to the area. The same growth form is repeated in a few
widely separated regions of the world where, although made up
of quite different species, it is a product of a similar complex of cli-
matic conditions. In the same way the vast expanses of deciduous
or coniferous forests in the temperate regions of the world are
each found where climatic characteristics fall within definite
limits, similar throughout.

General Climate and Vegetation Type.— Within the general

FlG. 9. Transition zones between stands of two life forms. The forest at
right (mostly buckeye) shows the usual gradual transition from a closed
stand to scattered, widely spaced individuals over a wide band— such as is
typical of most transitions from one community to another. The abrupt
transition from beech forest to grassland (at left) is unusual— Photo by D.
M. Brown.

vegetation type, certain variations may be expected. Species dif-
ferences are not uncommon although the growth form may be
uniform for all. Such differences are most pronounced when a
type of growth form extends over a wide latitudinal range. In the
arctic flora, which has an otherwise uniform physiognomy, the
number of species declines steadily northward. Within the grass-
land areas of the Middle West, there is obvious uniformity of
growth form from Canada to Texas, yet some species found in the
south are not found in the north and other species may be found
only in the north. Even those species that seem to range from one

30 THE STUDY OF PLANT COMMUNITIES * Chapter II

limit of a growth form to another may likewise have certain char-
acteristics, probably physiological, which limit the extent of their
area of favorable growth. Recently it has been shown that certain
grasses that seem to range throughout the latitudinal extent of the
prairie cannot be satisfactorily used to reseed northern areas when
the seed has been obtained in the south. Foresters, too, recognize
that it is advisable to replant with seedlings grown from locally
produced seed.

The more extreme (less favorable) the climatic conditions, the
less diversity can there be in the species and the fewer the species
will be because not many will have the adaptations necessary for
their survival. The numbers of species in a general vegetation type
are by no means constant throughout, especially nearing the limits
of the type. Here it might be expected that conditions would be
something less than optimum and that some species would be less
well adapted to the extremes than others. The same can be said for
numbers of individuals of a species. As conditions favoring a spe-
cies vary from their maximum, the number of individuals may be
expected likewise to fluctuate, and, near the limits of the range of
a growth form, the numbers of individuals of that growth form
would also decline. In the same sense, but in the opposite direction,
this marginal area would support a few species and individuals of
the contiguous growth form; thus transition zones between com-
munities are characteristic. Sometimes these transitions are wide,
sometimes relatively narrow, but rarely does one community,
large or small, have a sharp line of demarcation between itself and
its neighbor.

Local Habitats and Species Differences.— Climatic areas are of
considerable extent and usually include local diverse conditions of
soil or topography. Often these variations are so great as to result
in localized environments (habitats) quite unfavorable to the spe-
cies and even to the growth form of the region as a whole. Often
the conditions may be so much more favorable than those of the
general climate that a growth form from a neighboring region can
compete successfully. This is well illustrated by the trees and
shrubs extending far into the prairie along the streams, where the
favorable soil moisture is sufficient for them to compete success-
fully in a grassland climate.

NATURE OF THE COMMUNITY

A south-facing bluff forms a habitat that is almost always warm-
er and drier than the average for the region, while a north-facing
bluff is cooler and wetter. Barren exposures of rock or high, rocky
ridges represent one extreme in local habitats, while flood plains
of streams, lakes, and lake margins represent the other. Such habi-

FlG. 10. Aerial view of the forest that extends along the meandering
Sauris River far into the grassland of Nebraska —U. S. Forest Service.

tats are bound to support numerous species that are not character-
istic of the general climate and may even differ in their growth
forms. These local variations may be extremely restricted in area,
scarcely affecting the general physiognomic picture, as would be
true of the vegetation around a spring, or on a boulder in the
woods; but they may also be so extensive as to be misleading.
Cypress or cypress-gum swamps in some sections of the southern
states are so large that they might be viewed solely as a product
of climate, especially where little drained land is near supporting
upland vegetation. Many of the pine forests of New England and
the other northern states lie in a climatic region where spruce and
fir should eventually predominate. They are so extensively distrib-
uted that some ecologists recognize them as the ultimate growth
form for the region and as strictly controlled by climate, whereas,
their occurrence within the climatic area is closely associated with
light, sandv soils.

32 THE STUDY OF PLANT COMMUNITIES ' Chapter II

The kinds of plants, as to form and appearance, that can grow in
a climatic region are, therefore, determined by the overall climate.
The species within the general growth form may vary from place
to place or from one limit of the climatic area to another as de-
termined by local variations in some factors. Local habitats may
have such marked differences in growing conditions that not only
will species differ but even the growth form may not be that of
the climate of the region.

GENERAL REFERENCES*

J. Braun-Blanquet. Plant Sociology : The Study of Plant Communities.

A. E W SCHIMPER. Plant Geography upon a Physiological Basis.

E. Warming. O ecology of Plants.

J. E. Weaver and E E. Clements. Plant Ecology.

*See References Cited on page 362 and following for complete listings.

CHAPTER III

VEGETATIONAL ANALYSIS
QUANTITATIVE METHODS

FIXING THE CONCEPT OF A COMMUNITY
The fallacy of doing detailed physiological studies with an un-
named plant is obvious. If the physiologist does not know the
species with which he is working, his conclusions will be limited
to the particular group of plants he is using in his experiments.
The studies of taxonomists, floristic geographers, and geneticists
represent an accumulation of information and data upon which
the physiologist can draw and which he can use to make general-
izations and comparisons. All this information is connoted by the
scientific name of the plant being studied.

The ecologist, although working with communities, deals with
problems similar to those of the physiologist when he sets up the-
ories, attempts to find causes, to draw conclusions, or to formulate
laws. But the ecologist is faced with the necessity of determining
the make-up of the community with which he works before he
can proceed to an investigation of causes or to experimental con-
siderations. At present, most of the larger, regional, climatic vege-
tation types are so well known that their concepts are probably as
distinct to the ecologist as are those of most common species to
the taxonomist. For lesser communities, however, this is not true.
Furthermore, identification of such a community in terms of a
specific concept requires more than a superficial examination. Per-
haps an ecological classification of plant communities will never be
achieved with the same degree of perfection found in taxonomic
classification; perhaps such perfection is not necessary. It is neces-
sary, however, that there be means of characterizing a community
with sufficient accuracy to permit identification at any time, to
compare it with other similar communities, and to have an ade-
quate permanent record of its nature and occurrence. Undoubted-
ly, if such work is well done, it is justified on its own merits as a
phase of ecological investigation.

34 the study of plant communities * Chapter HI

If the major interest in a community is an experimental one and
the preliminary analysis and description of the vegetation have not
previously been made, the experimenter must first learn and re-
cord the characteristics of the community with which he intends
to work. Again, after experimentation or treatment, whether it be
of the community as a whole or of individual species, it often be-
comes necessary to evaluate the results in terms of the community
as a whole. There must also be a means of comparing the original
and the resulting communities at the beginning and at the end of
each experiment or treatment. The relationship of the individual
species to the community and the responses of the individual spe-
cies can best be interpreted when the constitution of the entire
community is positively established.

It is illogical to proceed with explanations when the subject it-
self is indefinite or unknown. Therefore, the first objective in
ecological work is to learn the composition and structure of the
community under consideration. Then, and only then, logically
follow a search for causes, experimentation, and interpretations
based upon a firm foundation.

QUANTITATIVE DATA A NECESSITY

In the early days of ecology, observation and description were
considered adequate for recording the characteristics of a com-
munity, but few observers see the same thing in the same way, and
few writers have the ability to translate exactly into words the
things they have seen. Thus, as in other sciences, ecology has be-
come more precise as it has developed and, with its concern for
greater detail, has demanded accurate measurement and precise
records of vegetation. This has led naturally to quantitative meth-
ods and terminology, which are becoming more uniform and,
therefore, more useful. Their use permits positive statements con-
cerning the numbers and sizes of individuals as well as the space
they occupy within a stand. With such data in hand, it is possible
to make comparisons of species or groups of species within a stand
or between stands. Likewise, the data constitute a permanent rec-
ord, which can be referred to again if the same stand or similar
stands are studied later. Also, as a permanent record, they are sub-

VEGETATIOXAL ANALYSIS 35

ject to reconsideration by other investigators, who may reinter-
pret them in the light of additional experience or information.

SAMPLING

The need for quantitative records has made it necessary to give
serious consideration to methods of sampling. Usually the mem-
bers of an entire community cannot be counted or measured, and
even if this were done, the information would be no more useful
or significant than an adequate set of data acquired by proper sam-
pling. Since this is true, it becomes of prime importance to deter-
mine what constitutes an adequate sample in terms of the commu-
nity as a whole and how to obtain such a sample with a minimum of
effort. At best, sampling for vegetational data is tedious and time-
consuming; often it may be extremely hard work. Nevertheless,
sampling conserves both time and labor as compared with an at-
tempt to analyze a whole community, and its results are much
more significant than those obtained by mere observation.

In this connection it should be emphasized that the early pro-
cedures of observation and reconnaissance are still of extreme im-
portance in determining where, how, and what to sample. These
activities are still a necessary part of community study although
they cannot be substituted for detailed analysis. They serve to
form a basis for theories or ideas that may in turn be substantiated
by quantitative evidence obtained by sampling. Preliminary recon-
naissance may likewise help to reduce the effort expended in sam-
pling. No sampling should be done without a thorough knowledge
of the history, physiography, and vegetation of the region as a
whole. Prior to sampling, the community should have been ob-
served repeatedly in different parts of its range and more particu-
larly under the varying local conditions where it exists. Finally,
the specific stand should be observed thoroughly to determine its
obvious variations, its extent, limits, and transitions to contiguous
communities. Then, knowing all this, together with the size of in-
dividual plants, the strata present, and the purposes for which the
sampling is to be done, one may plan his procedure in terms of the
desired results, the necessary degree of accuracy, and the time
available for doing the work.

Ecologists call a sample area or plot a quadrat, and the method

36 THE STUDY OF PLANT COMMUNITIES * Chapter 111

of sampling by the use of plots is commonly called the quadrat
method. The use of the sample plot is by no means restricted to
ecology, but its application in the sampling of natural vegetation
has led to methods peculiarly adapted to the ecologist's needs. The
quadrat has almost unlimited applications and has been used in a
great variety of ways.

Kinds of Quadrats.— The list-count quadrat is probably most
commonly used. With this the species are recorded and their num-

FlG. 11. A small quadrat laid out with meter sticks, which are pinned at
corners. Ready for list-count. This is a permanent quadrat that can be relo-
cated by paint markings on boulders. At Glacier Bay, Alaska, for the study
of early development of vegetation on raw morainic soil.75 Ice covered this
area thirty-seven years before picture was taken.— Photo by W. S. Cooper.

bers determined by count. This method is subject to many mod-
ifications depending upon circumstances. For trees, the individual
diameters might be recorded and later used for segregating size
classes, or perhaps for computing basal area (indicative of dom-
inance) for species. Bunch grasses, too, are often measured across
the base to obtain a basal area figure, which, combined with the
count, will give a better expression of the relative importance of
species. With herbs it is sometimes desirable to have additional in-
formation on the weight of tops, which must, therefore, be re-
moved for each species. In any event, the species are listed and
tabulated by number, weight, or size.
A chart quadrat is a more detailed record of the individuals

VEGETATIOXAL ANALYSIS

•S)

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t/5

<-»

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4-<

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-J j

*— '

(— *

C/5

• ^n

4->

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—

i—

4-1

5
u

t/5
4->

<**

- o

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r- '

r-1

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'lH

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cd

38 THE STUDY OF PLANT COMMUNITIES • Chapter HI

TRENCHING STUOY "2. SHORTLEAF PINE TYPE
CONTROL PLOT 9-12-34

A = Andropogon ternanuS
Ad = Aster dumosus
Ca = Catnenna angustifoiia
Cp = Ciadonia pyxidata
C = Ciador..a sylvatica
0 = Oicranum scoparium
Ds = Oicranum spunum
•Ov= Diospyros virginiana

0.35 «Pe
E = Eupatonum hyssopifolium
G = Grass seeding
H = Herbaceous seedling
L = Lespedeza repens
Oe = Oenothera longipedicellata
Ox = Oxahs stncta
P = Pamcum sphaerocarpon
Scale

»Pe = Pmus echinata

Pq = Psedera qumquefoiia

•Q = Quercus velutina
SB = Smilax bona-nox
S = Sohdago nemorahs
T b Thuidium dehcatulum

•U = Uimus alata
V = Viburnum afhne

0.66 leet

FIG. 13. The system of mapping used in the study illustrated in Figure 12.
Such a procedure is adaptable to many situations/"

146

present, giving their size and distribution within the area. This is
usually time-consuming, even on small quadrats with a relatively-
simple arrangement and few species. It does, however, permit
study at a later date— an advantage not to be ignored under many
circumstances. Small quadrats may be photographed with consid-
erable success if proper equipment can be brought to them con-
veniently. Camera stands of various sorts have been designed that
permit vertical views, and the photographs can be studied at leis-

VEGETATIONAL ANALYSIS

ure. Fairly accurate coverage for individual species can be deter-
mined from the prints with a planimeter (a mechanical device for
determining the area of a surface with irregular boundaries). Such
records are particularly useful when the areas are to be studied
over a period of time and when they are subject to treatment.

FlG. 14. A4apping a quadrat by the use of a pantograph, which reduces all
details to scale.— U. S. Forest Service.

When a high degree of accuracy is desired for small plots, a panto-
graph193 can be used with a drawing board, or sketching on co-
ordinate paper may be quite satisfactory, especially if the quadrat
itself is marked off into a grid pattern, as with strings. For small
quadrats of low or matted vegetation, a rigid frame permanently
rigged with fine cross wires to form a grid (see Fig. 12) can be
used to advantage since it can be moved from place to place, thus
saving the time of marking off" each new quadrat.25 Small quadrats
in relatively tall herbaceous vegetation or among shrubs and sap-
lings can be laid out more easily with rods or wooden strips cut to
proper length than with tapes (see Fig. 11). There are times when
the accurate measurement or recording of cover is too time-con-
suming or is not actually necessary. Estimation of cover merely
by inspection of each plot can be done with considerable accuracy
after only a little experience, and such an estimate may be suffi-
cient for the objectives.

THE STUDY OF PLANT COMMUNITIES ■ Chapter HI

The use of permanent quadrats has been advocated by many
ecologists, but few have followed their own excellent advice.
Whenever there is a remote possibility that a sampling area may
again be visited for further study, the quadrats should be marked
with permanent markers, for surprisingly worth-while results may

FlG. 15. Paired pictures illustrating slow development of vegetation on
»cks on Isle Royale. Lower picture taken seventeen years after upper.—

rocks on Isle Royale. Lower picture
Photos by W. S. Cooper™

be obtained by restudying identical areas after a period of years.
Such results are often valuable out of all proportion to the effort
required, especially when compared to the initial study. Most
quadrat studies are planned for immediate results and to help solve
problems of the moment, but with little extra effort they could be
used to yield returns over a period of years. Actually it would be
well to consider the possibility of making every quadrat permanent.
When Dr. W S. Cooper made his now widely known study
of vegetation on Isle Royale in Lake Superior, he photographed
his sampling areas and carefullv marked the spots even though he
had no definite plan for restudying the area. Seventeen years later

VEGETATIONAL ANALYSIS 44

he was able to relocate these points exactly, and he obtained a
striking series of matched pictures illustrating the development of
each of the vegetation types on the island.74 A number of similar
illustrations could be mentioned, but they are far too few.

Marking such plots when far afield may be something of a
problem, but by forehanded thoughtfulness combined with in-
genuity an adequate plan can usually be devised. A small can of
paint is no great burden when added to regular field equipment,
and its judicious use in conjunction with blazed trees, rock cairns,
or the like will usually suffice (see Fig. 11). It should be added
that experience indicates the advisability of recording in one's
notes a careful description of the markers and their exact posi-
tion with reference to landmarks of a permanent nature.

Quadrats originally set up for permanent study are usually of
an experimental nature. Perhaps they are to be subject to a treat-
ment of some sort, as, for example, different degrees of grazing,
watering, or thinning. For acceptable results these must always be
laid out in pairs so that an untreated plot can be used as a check
or control on the treated area. Usually it is desirable to replicate
the pairs one or more times, and this must be given serious thought
in terms of the extent of the stand and uniformity of conditions.
Such experimental areas are often established near at hand and in
easily accessible places, for they are to be visited regularly. With
plans made in advance, materials for permanent marking are among
the first equipment to be assembled. Substantial lengths of old pipe
or scrap metal, when driven into the ground leaving a few inches
protruding, are permanent and very satisfactory markers. If they
are painted conspicuously and marked with numbers, there can be
no confusion.

Experimental quadrats are of many types. Studies of competi-
tion and survival may involve thinning of stands, eliminating unde-
sirable species, or introducing other species, either by seeding or
planting seedlings, the object being to observe effects on the com-
munity or the introduced species. Newly exposed bare areas may
be studied to follow the natural development of vegetation, or
areas may be denuded and attempts made to produce artificial
communities. Perhaps the quadrats are used to evaluate the effects
of some controlled factor such as artificial watering or shading or

42 THE STUDY OF PLANT COMMUNITIES ■ Chapter HI

the application of a fertilizer. Again, animals may be the factor
under consideration, and then exclosures of the vegetation or the
animals will be necessary, depending upon objectives.84 Exclosures
should not be considered lightly, for their installation may require

VEGETATIONAL ANALYSIS 43

considerable time and labor. Also certain types of materials may
be surprisingly expensive, especially if plots are replicated. If the
effects of grazing are to be studied, a barbed-wire fence will keep
out cattle, but rabbits must also be considered. They may be at-
tracted by the very things that nourish within the exclosure after
the cattle are kept out. Again, small plots may be fenced for rab-
bits and yet permit squirrels or birds to come in over the top.
Then the entire plot must be covered. Lesser rodents may go
through or tunnel under the wire, and suitable precautions must
be taken to check them.

The effects of the exclosure itself upon the vegetation should
not be ignored since it may serve as a windbreak, which may re-
duce transpiration and intercept snow, soil, and seeds. Small plots
completely screened over will have quite a different micro-climate
from unscreened areas. To hold constant a single variable within
an exclosure is difficult, but it can be approached by having ex-
closures as large as possible, by insuring a liberal transition or isola-
tion strip around the margin, which will not be used in sampling,
and by having the barriers as low and as open as possible within
the limitations of the experiment.

Quadrat Methods —Actually the unit sampling area can be any
shape or size, and any number can be used in a variety of ways,
depending upon circumstances and objectives. As one soon learns,
the major concern is to get adequate data with a minimum of
effort. Because vegetation is so variable, generalizations cannot be
made to fit all situations. Because objectives are rarely the same,
methods quite satisfactory in one instance may not be so in another.
Set rules are not advisable for sampling, but certain generalizations
may well be considered in the light of experience.

Shape of Quadrat.— The term, quadrat, implies a square, and
this shape is undoubtedly more commonly used by ecoloo-ists than
any other. This is probably a matter of habit, for other shapes are
just as usable and sometimes more efficient. When Raunkiaer202
was making his pioneer studies of frequency, he at first used a
square frame for marking his sample areas but later used a circle
exclusively because of its convenience. He wished to have data
from many small quadrats that were randomly distributed. For
marking, he used a rod to which a stick was attached at right angles

44 THE STUDY OF PLANT COMMUNITIES * Chapter III

to form the desired radius. The rod was thrust into the ground at
the sampling point and rotated so that the stick marked the limits
of the circle to be sampled. He said, "The most convenient forms
and sizes of the unit areas are the best!' With low vegetation,
circular plots are sometimes a distinct advantage. An efficient
means of laying out circular plots is to use a set of hoops or rings
of proper size tossed in all directions from a central point. These
cannot be used in tall vegetation of any kind since they may be
obstructed when thrown or may be suspended above the ground.
Larger circular plots can be quickly and accurately marked with a
string attached to a free-turning ring on a central axis. Again, this
method will not be found satisfactory where vegetation is more
than waist high.

It has been demonstrated that a rectangular plot is significantly
more efficient in sampling than a square one of equal area since it
will tend to include a better representation of the variation in the
stand. Clapham,55 who worked with low herbaceous vegetation in
his study of this problem, concluded that plots lA x 4m. were the
most efficient in size and that to secure the same amount of infor-
mation with squares as with strips nearly twice as large an area
would have to be observed. Short strips (1:4) gave less variable
data than squares but more variable than long strips (1:16). The
same general conclusions were reached after studies of certain
types of sagebrush-grass range sampling.194

Size and Number of Quadrats —A. community is rarely homo*
geneous throughout as to species and their distribution. Newly
formed habitats, such as sandbars or tidal flats where often only a
single species is a pioneer, may support a nearly homogeneous
stand, but the usual community will have some variation. If there
were no variation, a single relatively small sample would always be
sufficient. Since variation is the rule, it becomes necessary to have
samples large enough or numerous enough to include the variation
and to have it fairly represented in the data. There is thus always
a question of how large and how numerous the quadrats should be
for adequate sampling.

The literature dealing with this problem is far too extensive to
review here. Agreement has not been reached on all phases of sam-
pling methods, and probably different methods will be advocated

VEGETATIONAL ANALYSIS 45

for some time to come. Several recent papers summarize ideas and
analyze the problem in detail. Their extensive bibliographies will
soon lead one to the conclusions of a variety of workers. Cain's
publications 43> 49 have done much to clarify methods of determin-
ing sample sizes and numbers both through his own contributions
of methods and their applications as well as through his summaries
of the literature. Penfound196 has brought together and analyzed
the usefulness of several currently favored procedures.

Species : area curves have been used in a variety of ways. Orig-
inally used by European ecologists to determine the "minimal
area" to be recognized as an "association individual" (^ stand), they
have been equally useful in arriving at numbers or sizes of plots to
be used in sampling individual stands. A characteristic curve will
result from plotting the number of species obtained against the
area sampled. The accumulated number of species found may be
expressed as a percentage of the total or as an absolute number and
plotted on the y axis. When the corresponding numbers of plots,
or sizes of area sampled, are plotted on the x axis, the curve formed
by the joined points will rise abruptly with first increases in area,
but will soon level off, and tend to rise only slightly thereafter
with increase of sampling area. It is assumed that the added infor-
mation represented in the slight rise of the curve is not sufficient
to justify the time and effort needed for the extra sampling. There-
fore, for this same type of vegetation, the sampling is assumed to
be adequate when the size of the sample somewhat exceeds the
area plotted against the point at which the curve flattens strongly.

It is of interest that, when the ratio of the x to the y axis is
shifted, it will result in a change in the form of the curve and a
consequent shift in the position where the curve tends to flatten.
This suggested the desirability of some means other than inspec-
tion for determining this point. Cain46 suggests that, in terms of
his experience, sampling is adequate when a 10 percent increase
in sample area results in an increase of species equaling 10 percent
of the total present. He suggests a mechanical means of determin-
ing this point on the curve regardless of the ratio of the x and y
axes. When a triangle is placed so that one edge passes through the
zero point and the point representing 10 percent of the area and
10 percent of the species, the triangle can be pushed upward along

46 THE STUDY OF PLANT COMMUNITIES * Chapter 111

a ruler placed at the right until its lower edge describes a tangent
to the curve. The point of the tangent is the center of the region
where the 10 percent relationship holds. If greater accuracy is de-
sired, the minimal area could be placed at the point of 5 percent
rise for a 10 percent increase in sampling.

By another procedure,49 the ratio of the x-y axes can be ignored,
a sample size or number can be selected, and a value set upon the
sampling. If the total number of species obtained in the sampling
is divided by the total number of sample units, the average incre-
ment of new species per additional sample unit is obtained. The
point on the curve is located (point A), in the region of which ad-
dition of a unit sample produces an increment of species equal to

Oak- Hickory Forest
piedmont of n.c.

MINIMUM NUMBER OF 10 X I0M.
FOR SAMPLING TREE STRATA

8 10 12 14 16 18 20 22

NUMBER OF 10 X 10 M. QUADRATS

24 26 28 30

Oak-Hickory Forest
piedmont of n.c.

MINIMUM NUMBER 0F4X4M. SHRUB
AND TRANSGRESSIVE STRATA

8 10 12 14 16 18 20 22

NUMBER OF 4X4M. QUADRATS

24 26

FlG. 17. Species : area curves for an oak-hickory forest, (A) indicating a
minimum of six 10 by 10 m. quadrats for sampling the arborescent strata, and
(B) a minimum of ten 4 by 4 m. quadrats for sampling the transgressive and
shrub strata. (C) A dune grassland community required a quadrat of hot less

VEGETATIONAL ANALYSIS

the average increment. Beyond this point addition of samples will
yield progressively less than the average. In the region of point B
a sample yields only one-half the information and at point C only
one-quarter the information obtained by a sample at point A.
Used in combination with the tangent procedure, this should be
helpful in interpretation of the species : area curve and the selec-
tion of numbers or sizes of plots most suited to a vegetation type.
If a series of quadrats of an arbitrarily set size is run in a stand,
a species : area curve constructed from the data will indicate how
many such quadrats would have been necessary for sampling to
achieve a desired accuracy. The most efficient size of plot to be
used can likewise be determined from the same preliminary series

Dune Grassland. N.C

MINIMUM QUADRAT SIZE IF 25 ARE USED

SIZE OF QUADRATS SQ.M.

Dune Grassland, N.C.

MINIMUM NUMBER OF ^ SQ.M.
QUADRATS

10 15

NUMBER OF i SQ.M. QUADRATS

than % sq. m. and (D) a minimum of six such samples. The lines tangent to
the curves were put in using Cain's triangle method described on page 000.
In (B), point a is equivalent to the average increment per sample, at point b
the yield is only one-half this increment, and at point c only one-quarter the
increment.

48 THE STUDY OF PLANT COMMUNITIES ■ Chapter III

of data if each quadrat is subdivided into successively smaller plots
(e.g. : 1, /z, 54, Vs sq. m.) for which the records are kept sep-
arately. The data obtained from the smallest area then become a
part of those for the next larger area, and so on. When the number
of species is plotted against increase in area sampled, the usual
curve is formed. The information regarding numbers and sizes of
10m 4m

a b e

FIG. 18. Nested quadrats. (A) shows a plan used successfully for sampling
the several strata in forest stands. (B) and (C) show systems of dividing plots
of any size for accumulating data to be used in determining the desirable size
of plot by means of species : area curves.

plots is then applied to sampling of similar or closely related com-
munities. The procedure for determination of numbers and sizes
of plots is well illustrated by Cain's study of sample-plot tech-
niques applied to alpine vegetation in Wyoming.49

When vegetation is stratified, a series of sample plots large
enough to include the trees will certainly be large enough for all
plants and strata. The work involved in measuring or counting the
lesser vegetation in such plots, however, would be unnecessarily
great. It, therefore, becomes advisable to sample each stratum sep-
arately with an appropriate size of plot for each. These plots can
be "nested" one within the other and the work thus materially
reduced. Sampling forest vegetation in the Piedmont area of North
Carolina has been done satisfactorily by using 10 x 10 m. plots for
trees, 4 x 4 m. plots for all other woody vegetation up to ten feet
tall, and 1 x 1 m. plots for herbs.183 By separating the data for
trees into overstory and understory individuals and by recording
separately those woody plants less than one foot tall and those
from one to ten feet tall, five strata were distinguished. More
might be necessary or advisable under other conditions.

VEGETATIONAL ANALYSIS

In general, it may be said that small plots require less work than
large plots, both in the laying out and in the obtaining of data,
even though more small plots than large ones are needed for com-
plete sampling. At the same time, there is a further saving of effort
in that the total area sampled by small plots may usually be less
than that sampled by large plots and yet give comparably valuable
information.

Distribution of Quadrats .—When the size, shape, and numbers
of quadrats have been determined, there still remains the question
of how they are to be placed efficiently and in such a fashion that
they will give representative data for the stand as a whole. If a
stand had a perfectly homogeneous composition, it would make no
difference where the sampling was done, but this is rarely, if ever,
true. Differences in the soil, drainage, and topography are usually
present and are reflected in the vegetation. These variations must
be fairly represented in the sample. It becomes necessary, there-

FlG. 19. The distribution of quadrats in a stand according to three differ-
ent systems. (A) Random distribution as determined by Tippett's numbers.252*
(B) Spaced as widely and evenly as possible by survey and measurement. (C)
Distributed evenly along lines run by compass or sighting; spacino- deter-
mined by pacing.

fore, to distribute the quadrats throughout the stand, and a plan
that will eliminate the human factor in placing the individual plots
is desirable.

The statistician prefers a sampling system that gives him data
obtained at random.216 This demands a division of the entire stand
into possible sampling areas and then a selection of actual sampling
areas determined strictly by chance. Under such conditions, the

50 THE STUDY OF PLANT COMMUNITIES * Chapter III

statistician is able to express mathematically how good his sam-
pling may be. Such a method frequently brings several sampling
areas into close proximity at the same time that wide areas are left
unsampled. Within these wide areas, there are very likely to occur
a number of infrequent or unusual species in small numbers, which
would be of little concern in a statistical treatment but whose
presence could be of great interest to the ecologist. For him, it is
usually desirable to have as many of the variations as possible rep-
resented in his data because they are subject to interpretation in
terms of experience and the nature of related communities. For
such purposes, statistical methods are often of little help. It is,
therefore, probable that quadrats distributed systematically
throughout the stand as evenly and widely as possible are quite
satisfactory for most ecological sampling. In fact, systematic sam-
pling is likely to be better than random sampling for certain eco-
logical purposes.

Any method that will insure wide and even distribution of sam-
ples should be satisfactory. The limits and extent of the stand must
first be ascertained, and sampling plans made accordingly. Once
the plan is made, it should be followed rigidly unless some previ-
ously unknown irregularity, like a swamp or an outcrop of rock,
should fall within a sample.

In small stands it is possible to plan a grid pattern and to sample
at regular intervals in this pattern. When stands are large but of
reasonable uniformity, it is common practice to run one or more
lines across the greatest extent and to space the quadrats evenly
along these lines. It would appear that the more widely the plots
are spaced in an area to be sampled the greater the efficiency of the
sampling unit, provided the spacing is not so great as to make
correlation negligible between adjacent plots.194 Under some con-
ditions, it may be desirable to run the lines with a surveyor's
transit, although a compass line will usually suffice, and in open
country it is possible to run them by sighting on some landmark.
The spacing may sometimes require accurate measurement, but
pacing may serve quite satisfactorily. The important thing is to
avoid any method bordering on personal judgment in placing the
plots once the sampling is under way. This should be remembered
particularly when the sampling is being done to prove or disprove

VEGETATIONAL ANALYSIS

a point. Under such conditions, there is often a strong temptation
to shift a plot a few feet or more to include or exclude a desired or
undesired species or condition.

FlG. 20. Diagrammatic profile along a transect on the dunes at Ft. Macon,
N. C. Physiographic-vegetational zones are indicated. Transect was 110
meters long and horizontal scale is one-half the vertical.

188

102 t 103

PEAK OF REAR DUNE

. FlG. 21. Portion of field-mapped transect along profile shown in Figure
20 from 97 m. through 104 m. across the transition from Zone 4 to Zone 5,
where dominance changes from Andropogon to Uniola. The symbols indi-
cate A—Andropogon, U— Uniola, H—Heterotheca, C—Cenchrus, Oe— Oeno-
thera, L—Leptilon. Such a map gives accurate quantitative data for each spe-
cies as well as a visual record of changes ki vegetation associated with habitat.
See Table 1.

Transects— A transect is a sampling strip extending across a
stand or several stands. It is most often used when differences in
vegetation are apparent and are to be correlated with one or more
factors that differ between two points. From a flood plain of a
river to the adjacent upland there would be marked changes in
moisture conditions, and in such a place a transect can be useful
for determining the range of moisture requirements of individual

52 THE STUDY OF PLANT COMMUNITIES * Chapter III

TABLE 1. Average density (D) and cover (C), by zones, of principal
species mapped on a transect from high tide to the crest of the rear dune at
Ft. Macon, N. C. (see Fig. 20). Both cover and density values show the pre-
dominance of Uniola in exposed zones and of Andropogon in protected ones.
This is correlated with salt spray.188

ones

Transect I

III

Ilia

Uniola paniculata L.

D
C

11.8

2.7

7.5
6.9

3.3
0.3

0.4
0.03

4.0
9.1

10.6
18.9

Andropogon littoralis Nash

D
C

2.1

2.2

2.5
7.1

5.7
7.3

6.6
15.4

4.1

5.5

0.9

2.3

Oenothera humifusa Nutt.

D
C

1.1
0.06

1.8

7.2

1.8

6.5
0.9

4.5

Heterotheca subaxillaris (Lam.)
Britt. and Rose

D
C

4.3
0.5

0.5
0.1

0.5

0.2

1.1

0.8

Leptilon canadense (L.) Britton

0.4

1.2

15.1

0.06

6.2

5.1

Euphorbia polygonifolia L.

0.1

0.3

0.2

Fimbristylis castanea (Michx.)
Vahl '

D
C

1.4
0.2

11.6
16.6

1.9
0.1

Myrica cerifera L.

D
C

0.5
14.7

species. Transects are also useful in altitudinal studies and in any
situation where transitions between communities occur.

Sizes of transects, just as sizes of quadrats, will be determined
by conditions. A transect reaching from one small community to
another, across a transition zone, might need to be only a few
meters long and perhaps a meter or less in width. Transects from
lake margins across the several marginal girdles of vegetation that
are usually present might be much longer. One reaching from
high-tide mark across seaside dunes might be several hundred
meters long. A study of the zonation of vegetation on the Sierra
Nevada was made by mapping a transect seven miles wide and ex-
tending across the mountain range for a distance of eighty miles.144

When it seems desirable to map an entire transect in detail, it is

VEGETATIONAL ANALYSIS

advisable to do so by blocks. Values for each block may then be
conveniently used as quadrat data, an additional means of analysis
and expression of results. A variation of the transect is the method
of sampling a unit area at regular intervals along a line. These inter-
vals may be determined by distance or altitude. Such records
taken on several lines are particularly helpful in mapping several
vegetation types that intergrade irregularly over an extensive area.
In the early land surveys of the northern and midwestern states, it
was required that the characteristic trees be listed in the records
for definite intervals along the lines run by the surveyors. Since
the county and township lines they established still stand, it has
been possible to reconstruct with considerable accuracy the com-
position of the forests as they then existed as well as the limits of

FlG. 22. Forest associations of southwestern Michigan as reconstructed from
the field notes of the old land survey. Unshaded areas, marked B, beech-
maple forest; X = hemlocks, constituting, along lake shore, a codominant
with beech and maple; O = white pines (a mark for each locality of occur-
rence noted in the survey); horizontally shaded areas, oak-hickory forest;
obliquely shaded areas, oak-pine forest; stippled areas, dry prairies; and ver-
tically shaded areas, swamp associations.— From Ke?Joyer.i3i)

54 the study OF plant communities ■ Chapter HI

forest and grassland.139 These surveyors' "transects" were some of
the first and longest ever run.

Sometimes there is an advantage in the use of "line transects" in
which the species are tabulated as they occur along a line. The
method is adaptable to the determination of numerical abundance,
frequency, coverage, and other characteristics. It has the advan-
tage of speed and apparently gives accurate information, consider-
ing the time it requires. It is particularly useful in dense stands of
scrubby vegetation, which would be very difficult to sample with
quadrats. Determinations of cover in dense chaparral using line
transects gave results that compared very favorably with those ob-
tained by complete charting, although the transects were made in
a small fraction of the time required for the detailed procedure.13

r-130

-loo

Rain Forest Swamp Forest

Fig. 23. Profile diagrams (bisects) of two types of tropical forest. Note
that difference in height of trees and in form of trunk is well shown and that
rain forest has three distinct strata of trees but swamp forest has essentially
one.— After Beard.15

Bisects.— These are variations of transects in that they are sam-
ple strips aiming to show the vertical distribution of vegetation.
Thus they may include stratification and layer communities from
dominant trees to seedlings on the forest floor and, in addition,
show the stratification and root distribution of these same plants
below ground.

GENERAL REFERENCES

S. A. CAIN. The Species- Area Curve.

S. A. CAIN. Sample-Plot Technique Applied to Alpine Vegetation in Wyo.
F. X. Schumacher and R. A. Chapman. Sampling Methods in Forestry
and Range Management.

CHAPTER IV

VEGETATIONAL ANALYSIS
PHYTOSOCIOLOGICAL OBJECTIVES

The interest of European workers in community structure,
their desire to describe communities precisely, and their concern
with systems of classifying communities resulted in the develop-
ment of a phase of ecology known as phytosociology. Its develop-
ment was paralleled by (1), the growth of systems of terminology
with which the characteristics of a community could be adequate-
ly expressed, and (2), the testing and refinement of methods for
obtaining quantitative data on the structure and composition of a
community to support the systems of description.

Phytosociological methods and terminology have become pro-
gressively more standardized, but, as yet, there is not complete
agreement among workers. The problems to be resolved are still
of the same nature as those of earlier days as is illustrated by a re-
cent characterization,196 which groups them into two categories :
(a) the size and number of quadrats to be utilized and (b) the
conditions to be investigated. The first we have discussed at some
length as a part of quantitative methods in community analysis. It
should be remembered that the development of these methods has
been strongly influenced by phytosociological interests. Although
the quadrat method in ecology had its origins in America, its
adaptation and refinement for complete analysis and description
of communities must be largely credited to European workers.

What phytosociological values are necessary for an adequate
characterization of a community would hardly be agreed upon by
all workers even today. Through the years this has been the subject
of much debate. Some early workers attempted to describe com-
munities on the basis of a single value (e.g., frequency) for each
species. Today such a simple system would not be recommended
by anyone, and, regardless of objectives, several values are now
used in all phytosociological analyses. Adethods of sampling and
objectives have always influenced each other, and, therefore, it is
not surprising that early European workers had widely different

56 THE STUDY OF PLANT COMMUNITIES * Chapter I V

approaches, which led to somewhat different conclusions. Several
centers of thought and research naturally grew up, which still in-
fluence our thinking and procedure. The ideas of the so-called
Zurich-Montpellier school have gained rather wide favor, largely
through the influence of Dr. J. Braun-Blanquet,34 and they will be
summarized in the remaining section of the chapter.42

Before proceeding with this summary, it seems entirely appro-
priate to point out the unfortunate fact that Americans have been
slow to adopt the phytosociological approach, probably because
of a lack of appreciation of the usefulness of sociological data. Al-
though phytosociology is, in itself, only a phase of ecology, its
methods are useful far beyond the field for which they were de-
veloped. Whenever communities must be described or the sig-
nificance of individual species in a community must be evaluated,
phytosociological concepts and methods are applicable and usu-
ally with distinct advantages. This means that the methods are
useful in experimental studies of communities, for comparing one
community with another, for showing changes in a community
from year to year, and, in fact, whenever precise information is
needed about community structure and the part contributed by
various species. Its possible applications are almost unlimited. To
illustrate, various of its methods have been used to advantage in
such diverse problems as correlating the progressive changes of
vegetation and soil on abandoned fields,20 showing the effects of
different intensities of fire on the structure of pine stands,1S4 and
for demonstrating differences in virgin forest with changes of

topography.187

STRUCTURAL CHARACTERISTICS

The sociological characters of an individual stand or concrete

community may be conveniently grouped in two categories :
quantitative and qualitative. Quantitative characters, obtained by
quadrat methods, indicate numbers of individuals, their sizes, and
the space they occupy. Qualitative characters indicate how species
are grouped or distributed, or describe stratification, periodicity,
and similar conditions, and are based upon the knowledge derived
from long familiarity and observation of the community.

Quantitative Characters. — Numbers of Individuals. — Under
some circumstances, it mav not be practicable to make actual

VEGETATIONAL ANALYSIS

Table 2. Portion of a list of species occurring on the east coast of Greenland at
fourteen localities ranging from (A) 70° N latitude, southward to (N) 65° N latitude.
Both presence and degree of importance of the species in each locality is indicated
by the field-assigned numbers according to the following scale:

5 — very common (important constituent of several closed communities); 4 —

common (more scattered occurrence); 3 — here and there; 2 — uncommon; 1 — rare;

-\ present.

The listed species were selected to show how the system of values indicates range
limits and progressive changes of importance with latitude. From Bocher.24*

Localities

E
4

H
4
4

/
4

K
4

Cystopteris fragilis

3
4
4

}

Cerastium alpinum

Minuartia biflora

Silene acaulis

5
5

Sedum roseum

3
5

Oxyria digyna

Polygonum viviparum

Salix herbacea

Potentilla tridentata

Polystichum lonchites

Alchemila filicaulis

Sagina intermedia

Draba rupestris

Empetrum nigrum

Salix arctophila

Epilobium arcticum

3
3

Potentilla pulchella

Ranunculus sulphur eus

Draba lactea

Dryas octopetala

Draba alpina

Cassiope tetragona

58 THE STUDY OF PLANT COMMUNITIES • Chapter IV

counts, but plentifulness may rapidly be estimated according to
some scale of abundance similar to the following :

1. very rare

2. rare

3. infrequent

4. abundant

5. very abundant

Such estimates are particularly useful when several similar stands
of uniform composition are to be surveyed within a limited time.
Assuming the sampling is adequate, the determination of actual
numbers by counting is of greater value because it permits the ex-
pression of density, which is abundance on a unit-area basis.

Density is the average number of individuals per area sampled.
Since it is an absolute expression, the significance of density in in-
terpretation may be overemphasized unless one remembers that it
is an average value. Not all species with equal densities are of equal
importance in a community, or need they be similarly distributed.
If ten individuals of a species are counted on a series of ten plots,
the density is "one" regardless of whether they are all found in
one plot or one in each of the ten plots. It becomes necessary,
therefore, to interpret density values or to specify other charac-
ters that, combined with density, serve to complete the picture.
One such value is frequency.

Frequency — This value is an expression of the percentage of
sample plots in which a species occurs. In the example above, the
plants that were all found on a single plot would have a frequency
value of 10 percent, whereas, if they had occurred in every plot,
the value would be 100 percent. Thus frequency becomes a very
useful value, when used in combination with density, for then not
onlv the number of individuals is known but also how they are
distributed in the stand. These two characters are of prime impor-
tance in determining community structure and, taken together,
have a variety of uses far beyond those of other quantitative values.
The use of frequency as a single determination in analytic pro-
cedure has proven unsatisfactory, although numerous attempts
have been made to show its adequacy.

It should be emphasized that frequency values cannot be com-

VEGETATIONAL ANALYSIS

PINE

HARDWOODS

39.6-

Density-Frequency

20.0-

10.0-

Overstory

Understory

Fig. 24. Bar diagrams of density, frequency, and basal area to compare
pine and hardwood development in an unburned pine stand (A), with por-
tions previously subjected to surface fire (B), and crown fire (C). Densities
are indicated by the height of the columns above the zero line and frequen-
cies by the width of the columns. Basal areas in square feet are indicated by
the length of the columns below the zero line, and the width of these
columns indicates percent of total basal area in the stand. Values for density
and absolute basal area were modified by the factor 2 ^j~y because of their
wide range.

184

]*\red unless determined with plots of equal size. The larger the
pots, the higher the frequency.

'*§ Frequencies may conveniently be grouped into classes, for ex-
ample, A 1-20%, B 21-40%, C 41-60%, D 61-80%, E 81-100%.

60 THE STUDY OF PLANT COMMUNITIES • Chapter IV

Raunkiaer202 used these five classes and, on the basis of more than
eight thousand frequency percentages, found that Class A included
53 percent of the species; B, 14 percent; C, 9 percent; D, 8 percent;
and E, 16 percent. From these data he drew his "Law of Fre-
quency" which states that Class A>B>C|D<E. This led
to numerous investigations to check on the validity and univer-

TABLE 3. The effect on frequency of increasing size of quadrat as illus-
trated by data on Alpine fell-field vegetation in the Rockies. Quadrat sizes
in sq. m. From(4y).

Arenaria sajanensis . . .

Selaginella densa

Trifolium dasyphyllum
Eritrichium argenteum .
Sieversia turbinata ....
Polemonium conjertum .

Phlox caespitosa

Sedum stenopetalum . . .
Paronychia pulvinata . .

Silene acaulis

Potentilla nelsoniana . .
Potentilla quinquefolia.

Potentilla sp

Polygonum bistortoides .
Artemisia scopulorum .

Sieversia ciliata

Arenaria macrantha. . .
Erigeron compositus . . .

Total species

Average frequencies . .

1 1 10

100
100
80
80
50
40
30
30
30
20
20
20
10
10
10

i/4

100
100
100
80
50
40
50
50
30
30
20
20
20
20
10
10

16
45.6

■12

100
100
100
90
60
40
50
50
50
30
30
20
30
20
20
20

48.7

i/i

100
100
100
90
80
50
60
60
50
70
30
30
30
20
20
30
10
10
18
52.2

sality of the principle of frequency distributions in plant com-
munities.137 The results have been in essential agreement regardless
of the vegetation type. Class A will normally be very high because
of the numerous sporadic species to be found with low frequency
in most stands. Class E, and to a lesser extent D, must always bi
relatively high because of the species that dominate the commu-

VEGETATIONAL ANALYSIS

nity. If quadrats are enlarged, classes A and E will enlarge and the
lesser classes will decrease accordingly. Frequency classes, there-
fore, are comparable only when based upon samples of the same
size.

A frequency diagram is useful in indicating the homogeneity of
a stand since floristic uniformity varies directly with the values for

ABODE

Raunkiaer's

NORMAL

ABODE

Kenoyer's

NORMAL

15 YR.

34 YR

90 YR

abode abode abode

Loblolly Pine Stands-3ages

_□

ABODE abode abode abode abode

Five Stands Virgin Red Fir- Sierra Nevada
FlG. 25. Frequency diagrams of pine stands of different ages and of virgin
red fir stands compared with Raunkiaer's and Kenoyer's normals. The pine
stands were all relatively homogeneous but became slightly less so with age
as the total number of species increased by 25 percent and the accidentals de-
clined. Class E, the dominants, remained essentially constant throughout the
series. All the virgin red fir stands were extremely homogeneous in spite of
a high proportion of incidentals occurring sporadically. The stands were also
similar to each other although widely distributed along the Sierra.

classes A and E. When classes B, C, and D are relatively high, the
stand is not homogeneous. In general, the higher Class E may be,
the greater the homogeneity.

Cover and Space.— Although density and frequency indicate
numbers and distribution, they do not indicate size, volume of
space occupied, or amount of ground covered or shaded. These
characteristics are desirable additional values that contribute ma-
terially to an understanding of the importance of a species in a
stand, since they are closely related to dominance.

As suggested under Quadrat Methods (Chap. 3), cover can be

62 THE STUDY OF PLANT COMMUNITIES • Chapter IV

estimated with some success or may be accurately determined by
various devices for measurement and recording. When vegetation
is stratified, the cover must be considered in terms of the stratum
to which the species belongs. For rapid estimation, as well as for
analysis of results, there is a distinct advantage at times in using
several cover classes. Braun-Blanquet recommends five :

1. covering less than 5% of the ground surface

2. covering 5% to 25%

3. covering 25% to 50%

4. covering 50% to 75%

5. covering 75% to 100%

In studies of grassland, estimates and measurements of cover are
extremely useful because the variations in size and form of grasses
make counts difficult 'and of little value. For expressing cover,
sometimes as area of coverage, sometimes as basal area of clumps,
range ecologists frequently use the term, density . This usage is,
of course, at variance with the phytosociological application and,
consequently, leads to confusion of interpretation unless it is
known, for example, that a "density list"96 applied to grassland,
refers to area or cover for each species, and that "square foot
density"247 also indicates coverage evaluated by a different method.

Determination of the volume of space occupied by species is
difficult and has not been widely done. When all plants are small,
cover alone serves very well, especially when strata are distin-
guished. With grasses, as in pasture studies, clipping and weighing
the tops is sometimes necessary for accurate comparisons. In for-
est studies, the estimate of volume of standing timber as used by
foresters can be used to advantage, but a more useful value is basal
area. Diameters can be determined accurately and quickly with a
diameter tape, and basal area, easily obtained from standard tables,
can add much to an evaluation in terms of size and bulk that can-
not be visualized through the other quantitative characters. This
provides a particularly useful means of comparing the relative im-
portance of species of trees and, in addition, permits analysis in
terms of size or diameter classes among the sapling and understory
individuals. Several quantitative characters can be advantageously
combined in the form of phytographs (Fig. 26) for evaluation.

VEGETATIONAL ANALYSIS

SPRUCE
FLAT

SPRUCE
HARDWOODS

OLD FIELD
SPRUCE

SPRUCE
SLOPE

Picea
rubens

Abies
balsamea

Betula
lutea

Betula
papyrif era

Acer

rubrum

Fraxinus
americana

Acer

saccharum

Sorbus
americana

FIG. 26. Phytographs showing the relative importance of the dominant
species of trees in four types of pulpwood forest in northwestern Maine.
Radius 1, percentage of total dominant abundance; Radius 2, percentage fre-
quency; Radius 3, percentage of total size classes represented; Radius 4, per-
centage of total dominant basal area. The inner end of each radius represents
the absence of its assigned sociological value.191

64 THE STUDY OF PLANT COMMUNITIES ■ Chapter IV

Qualitative Characters.— These characters, which include socia-
bility, vitality, stratification, and periodicity, are mostly not de-
rived from quadrat studies but from observation of, and wide ex-
perience with, the community. They describe the plan and organ-
ization of its components, which have been evaluated previously
in terms of measurements and counts. When the quantitative an-
alysis has been fairly complete, especially including density or
cover in conjunction with frequency, and when strata have been
analyzed separately, the qualitative characters are already largely
included in the quantitative picture.

Sociability .—This character evaluates the degree that individuals
of a species are grouped or how they are distributed in a stand. It
has also been expressed as gregariousness or dispersion. Each of the
various scales used to indicate degree of sociability include expres-
sions which range from plants occurring singly, as one extreme,
through intermediate conditions (patches, colonies, or groups), to
large colonies, mats, or pure stands at the opposite extreme.

The sociability of a species is not a constant, for it is determined
by the habitat and the resulting competition of the species with
which it is associated. Since habitat conditions are not constant and
since communities change, especially in plant succession, the so-
ciability of a species, even in the same locality, may change con-
siderably.

Dispersion is a statistical expression that has been applied to
sociability. If dispersion is normal, it implies a randomized distri-
bution such as might be expected by chance. In hyperdispersion
there is irregular distribution, which results in crowded individ-
uals in some areas and their complete absence from others. Hypo-
dispersion means that the arrangement is more regular than would
be expected by chance, as, for instance, the plants in a cornfield.
All of these conditions are recognizable in natural communities
and, when density-frequency values have been determined, are
noticeable in the data.

Vitality.— Not all species found in a given stand need belong to
the community. Unless the plants are reproducing, they are not
completely adapted to the conditions and may disappear entirely.
Even species constantly present in a community mav be derived
from seeds produced elsewhere and transported by wind or some

VEGETATIONAL ANALYSIS 6S_

other agency. It becomes necessary, therefore, to know something
of the vigor and prosperity of the species before classifying it as
a true community member.

Vitality need not always be listed for all species, but it must be
considered in evaluating their importance, whether it is done sys-
tematically or not. Vitality classes or degrees of vitality include :
( 1 ) ephemeral adventives, which germinate occasionally but can-
not increase, (2) plants maintaining themselves by vegetative
means but not completing the life cycle, (3) well-developed plants,
which regularly complete the life cycle.

Changes in the vitality of species are often indicators of com-
munity change or plant succession. Dominants decreasing in num-
bers and reproducing feebly indicate future radical changes. Rap-
idly increasing numbers of a species previously of little importance
may suggest the new dominants to come.

Stratification— The necessity for recognizing the strata of a
community becomes obvious when sampling is attempted. The
several strata that may occur were described under sampling pro-
cedure. Diagrams of stratification combined with cover are often
used effectively to show the relative significance of the several lay-
ers in a stand. The physical and physiological requirements of spe-
cies in different strata can be appreciated fully only when the
stratification both above and below ground is clearly worked out.
Then the micro-environments of these strata may be considered in
terms of cause and effect.

Periodicity .—The conspicuous rhythmic phenomena in plant
communities are those related to seasonal climatic change, and, of
these phenological changes, the obvious ones have been given most
attention. Flowering and fruiting^ periods have been noted for so
long that they are fairly well known; in fact, phenology is often
thought of as referring only to these phenomena. In community
studies the terms aspect dominance and seasonal dominance have
been used to describe situations in which a species or group of
species appears to be dominant for a portion of the year, usually
because of conspicuous floral characters.

Of equal importance to the community is the seasonal develop-
ment of vegetative parts. The seasonal aspect of the individual may
proceed through several phases, including a leafy period, a leafless

66 THE STUDY OF PLANT COMMUNITIES « Chapter IV

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VEGETATIONAL ANALYSIS

period, a flowering period, a fruiting period, an embryo period,
and perhaps others. Rarely will all the species of a community
have these periods strictly coinciding. Consequently, in temperate
climates, the community as a whole usually has seasonal aspects,
which are termed vernal, estival, autumnal, and hibernal. The
structure and species of a community are strongly influenced by

FlG. 28. Aspect dominance as illustrated by chandelier cactus (Opimtia
arbor -esc ens) in a mixed prairie community (Bouteloua-Hilaria). El Paso
County, Colo. The cactus makes up only 8.9 percent of the total cover.—
Photo by R.B. Livingston.

the extent to which periodic phenomena in the individuals are
adjusted to each other.

Light and moisture conditions on the floor of a deciduous forest
during the vernal period permit the growth and maturation of nu-
merous herbs before the estival period. When the trees and shrubs
are in full leaf, these herbs are already declining to a fruiting or
resting condition and are unaffected by the reduced light and
moisture available to them. These vernal herbs are a part of the
community and must be so considered.

Another illustration of a periodic phenomenon that may be im-
portant in sociological relations is the time of growth. Height
growth has been systematically studied for numerous woody spe-

68 THE STUDY OF PLANT COMMUNITIES * Chapter IV

cies, but the periods of root elongation are rarely known. Studies
of loblolly pine205 showed, surprisingly, that it makes some root
growth in every month of the year. Even in the winter months,
its roots are constantly coming in contact with new supplies of
soil water, which fact may partially explain its ability to thrive in

FlG. 29. Vernal aspect dominance of atamasco lily (Zephyranthes atamas-
co) in a low North Carolina meadow where only grasses, rushes, and sedges
are visible a few weeks later.— Photo by H. L. Bloniquist.

the southeastern states where transpiration may at times be fairly
high during the winter.

Periodicity may be controlled by a variety of factors. Length
of day affects the time of flowering, some species requiring long
days, some short. The fall of leaves in autumn is a response not to
temperature but to length of day. Desert vegetation may flower
or not depending upon precipitation, and semidesert plants reg-
ularly flourish during the brief seasonal rains and exist in an almost
dormant condition for the remainder of the year. Arctic and alpine
areas usually receive little rain. The melting snow provides the
moisture for vegetation. In situations where little snow accumu-
lates or where it melts and disappears quickly, the vegetation is

VEGETATIOXAL ANALYSIS 69

sparse and takes on a hibernal aspect very quickly. Where snow
patches remain well into the summer and provide a water supply
by melting gradually, the estival aspect may carry on for several
weeks after plants in less favorable sites near by have gone to seed.
Plants deeply buried under snow may not be exposed until so late
in the season that conditions are unfavorable for flowering, and, as
a result, they produce no fruit or seed.

SYNTHETIC CHARACTERISTICS

It has previously been pointed out that it is often desirable as
well as practical to consider a community in the abstract as well as
in the concrete sense. When a community is studied on this basis,
it becomes necessary to observe numerous stands and to determine
whether they actually do belong to the same community and to
what extent they vary from each other. It is desirable also to know
which species, singly or in combination, may be taken as indica-
tors, which species are only incidental, which ones are always
present, and which ones occur only when a stand develops under
a given set of conditions.

Thus, for a complete synthetic analysis, it is desirable to have in-
formation on as many stands as possible, or at least enough stands
to be representative of the whole. These should be distributed
throughout the range of the community and under all the variety
of conditions in which they develop. Again, to make a proper an-
alysis, only those stands should be employed that are in a com-
parable stage of development or maturity and that are extensive
enough to include all the important species and most of the antici-
pated variations.

Presence.— A most useful synthetic character involves merely
the degree of regularity with which a species occurs in the stands
observed. When the species present in each of the stands have
been tabulated, the presence of each is expressed by the percentage
of stands in which it occurred or by a five-degree scale of presence
classes.

1.— rare (1-20% of the stands)

2— seldom present (21-40%)

3. -of ten present (41-60%)

4— mostly present (61-80%)

5— constantly present (81-100%)

70 THE STUDY OF PLANT COMMUNITIES • Chapter IV

The number of stands necessary for a study of presence as well
as the necessary extent of stands cannot be arbitrarily stated. Ma-

TABLE 4

Species

X
X
X

X
X
X
X
X

X
X
X

X
X

X
X
X
X

X
X
X
X
X
X

X
X
X
X

Trees
Abies magnified

X
X

X
X
X

X
X

X
X
X

Pinus monticola

Pinus contorta

Tsuga mertensiana

Abies concolor

Acer glabrum

Shrubs
Ribes viscosissimum

X
X

X
X
X

X
X

X
X
X

X
X

X
X
X
X
X
X
X
X
X
X
X

X
X

X
X
X
X

X
X

X
X
X
X
X

X
X

X
X
X
X

X
X
X
X
X
X
X
X

X
X
X
X

X
X
X

X
X
X
X

Symphoricarpos rotundifolius

Ribes montigenum

Sambucus racemosa

X
X
X
X
X
X
X
X
X
X
X

X
X

X
X
X
X

X
X

X
X
X
X

X
X
X
X
X
X

X
X

X
X
X
X

X
X

Ribes cereum

Spiraea densiflora

X
X
X

X
X

Arctostaphylos nevadensis

Symphoricarpos mollis

Lonicera conjugialis

Quercus vaccinifolia

Amelanchier alnifolia

X
X
X
X
X
X

X
X

X
X
X

X
X

Rubus parviflorus

Herbs

Chrysopsis breweri

X
X
X

X
X
X
X
X
X
X
X
X
X
X

X
X

X
X
X

X
X

X
X
X

X
X

Monardella odoratissima

Gayophytum ramosissimum

Pedicularis semibarbata

X
Y

Pirola picta

Phacelia hydrophylloides ■.

Poa bolanderi

X
X

Arabis platysperma

Corallorrhiza maculata

Thalictrum fendleri

Kelloggia galioides

Erigeron salsuginosus var.

angustifolius

X
X

Uxeracium albifiorum

Lupinus andersoni var. fulcratus . . . .

Viola purpurea

Chimaphila umbellata

X
\

X
X

Pentstemon zracilentus

Pla^iobothrys hispidus

VEGETATIONAL ANALYSIS

ture, homogeneous, undisturbed stands of virgin forest would re-
quire the observation of only small portions of individual stands
and a relatively small number of stands to give dependable infor-
mation. In younger, less stable vegetation, more stands and a wider
observation would be necessary so that variation would be repre-

€0

Presence

Frequency

Constance

12345 ABCDE 12345

FlG. 30. Presence, frequency, and Constance diagrams for Sierran red fir
forest, based on sixteen stands. The presence diagram is normal, especially in
the absence of a second maximum. The Constance diagram is constructed
from regular quadrat data rather than a Constance sample. Compared to a
frequency diagram it should show a material decrease in Class 1 because of
the greater odds on discovery of a single plant of an accidental species in a
restricted area. Surprisingly, with only forty species, it retains the same form
as the presence diagram (ninety-seven species) although the high Constance
classes are reduced. The frequency graph is normal, and indicates stands of
relatively great homogeneity.

sented and so that those species seldom present or rare would fall
into their proper classes. What this minimal area should be and
what the minimum number of species might be for the community
must largely be determined by experience and familiarity with the
community.

Constance.— When a unit area in each stand instead of the en-
tire stand is used for listing species, as for presence, the values are
termed Constance. There is thus no fundamental difference be-

TABLE 4. Portion of a presence table compiled from sixteen stands of vir-
gin red fir (Abies magnified) forest in the Sierra Nevada. Only Abies mag-
nified and Pinus monticola, of the trees, are constantly present (Class 5).
Only one shrub, Kibes viscosissimum, is a constant, others falling in Class 3
or lower. Five herbs are constants, eight are mostlv present (Class 4), and
five are often present (Class 3). Eleven herbs of Class 2 (seldom present) and
46 of Class 1 (rare) are not listed.189

72 THE STUDY OF PLANT COMMUNITIES • Chapter IV

tween presence and Constance. The latter has the advantage of
eliminating discrepancies resulting from sampling stands of un-
equal size. The lower classes of Constance are more uniform than
those of presence, for the larger the area examined the greater the
number of incidental species encountered.

Constancy bears a relationship to the abstract community very
similar to that of frequency in the concrete community. The prob-
lems of minimal area are similar and can to some extent be reduced
by the use of species : area curves as used in frequency determina-
tions. Both concepts are concerned with homogeneity, the one
with that of the stand, the other with that of the abstract com-
munity. If Constance values are divided into five classes and these
are diagrammed as for frequency, the results are quite different.
Instead of two maxima as in frequency, only the classes represent-
ing irregular occurrences are high, and each succeeding higher
class is apt to include fewer species.

Fidelity. — This character is indicative of the degree with which
a species is restricted to a particular kind of community. Species
may be grouped into five fidelity classes.

Fid. 1 .—Strangers, appearing accidentally

Fid. 2.— Indifferents, without pronounced affinity for any
community

Fid. 3.— Preferents, present in several communities but pre-
dominantly in one of them

Fid. 4.— Selectives, found especially in one community but
met with occasionally in others

Fid. 5.— Exclusives, found completely, or almost so, in only
one community

Species with fidelities 3-5 are termed characteristic species in a
community. Positive establishment of which species are character-
istic is possible only after all communities of a region have been
studied sociologically. Approximations can, of course, be made by
those of wide experience, but even then the assigned values must
be considered with skepticism. When fidelity values are accurately
determined, they contribute strongly to the recognition and classi-
fication of a community. However, studies of this sort have been
so few in the United States that it will be a long time before suf-

VEGETATIONAL ANALYSIS

TABLE 5. A summary of sociological concepts that permits presentation
of the important data for a community in a single tabulation. The quantita-
tive data (1) are derived from quadrats; the analytic data (A) from the study
of some one community; the synthetic data (B) from the study of several
different examples (stands) of the same community.— After Cain.*2

SOCIOLOGICAL SUMMARY

Organization

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ficient data have accumulated to permit accurate statements of fi-
delity for species of most communities. Under such conditions, it
seems advisable to use Constance, an absolute value determined
within the community in question, as a means of fixing upon the
sociologically important species. Some ecologists consider con-
stance of greater significance than fidelity for this purpose. It
should be noted that characteristic species are more responsive to
habitat variations and are consequently of greater indicator sig-
nificance than are, in general, the species of high Constance. It

74 THE STUDY OF PLANT COMMUNITIES * Chapter IV

would be desirable if we had both values available for all com-
munities.

Coefficient of Community.— When comparing two communi-
ties or the vegetation of two regions, a mathematical expression of
the similarity of lists of species may be useful. If community X
is compared to Y, the number of species common to both, ex-
pressed as a percentage of the number for Y has been termed the
coefficient of community. The same principle can be used for
evaluating variation or similarity among several stands of an ab-
stract community. Then, however, each must be compared with a
standard or list of the characteristic species of the community as a

whole.130

OBJECTIVES DETERMINE PROCEDURE

If these several sociological concepts are grouped systematically
in tabular form, their relationships become clearer (Table 5). Such
a grouping has the further usefulness of presenting tabulation of
values obtained in the field in compact and logical order for in-
terpretation.

When the objective is merely to describe a community as com-
pletely as possible, it might well be desirable to have such a table
completely filled out. In studies involving the application of phy-
tosociological methods to special problems it is frequently onlv
necessary to use a few of the values. This does not mean that not
all are of significance, or that some can be ignored entirely. Rather,
it suggests that each has its uses and that some are applicable where
others are not.

The limitations and possibilities of usefulness of the several con-
cepts become increasingly understandable after one has had some
experience with them. Nevertheless, selection of the most useful
values for study and application to a particular problem always
remains a matter for serious consideration. The concepts to be
used must be selected in terms of their contribution to the object
of the study, the time available, and the labor involved.

GENERAL REFERENCES

J. Braun-Blanquet. Plant Sociology : The Study of Plant Communities.
S. A. CAIN. Concerning Certain Phytosociological Concepts.
C. RAUNKIAER. The Life Forms of Plants and Statistical Plant Geography;
Being the collected papers of C. Raunkiaer.

Part 3 • Factors Controlling the
Community: the Environment

Vegetational analysis gives the information necessary to de-
scribe and name a community and provides data that can be used
to compare it with other communities or with itself after a lapse
of time or an experimental treatment. This in itself is worth while,
but the ecologist has the added objective of correlating the vege-
tational record so obtained with the environment. To interpret the
vegetational statistics, and to explain them in terms of cause and
effect, leads to an analysis of the environment and its relationships
to the community.

Since the environment consists of many factors interacting upon
each other and upon the vegetation, its complexity prohibits con-
sideration of it as a whole. The interactions are by no means all
clearly understood and the effects of a single factor upon an or-
ganism may be inadequately known; therefore, it is logical to ap-
proach the subject of environment through individual factors and
their effects. With information as complete as possible on the
operation of individual factors, explanations may often be found
for plant responses among the interactions and effects of a few of
the variable factors. The chapters of this section deal successively
with climatic, physiographic, and biological factors as each may
operate in the complex of factors termed environment.

CHAPTER V

CLIMATIC FACTORS: THE AIR

GASES OF THE ATMOSPHERE

The air surrounding the earth is made up of only a few gases in

proportions that remain remarkably constant. The average volume

percentages of dry air are : nitrogen, 78.09; oxygen, 20.95; carbon

dioxide, 0.03; and argon, 0.93. In addition, there are minute but

76 THE STUDY OF PLANT COMMUNITIES ■ Chapter V

measurable quantities of several rare gases, which have no part in
our discussion. Within the limits of the atmosphere that can affect
plants directly, there is but slight variation in the proportions of
these gases whether over the ocean or land, at sea level or on high
mountains. Minor but rather consistent variations have been found
over large industrial cities where quantities of carbon dioxide are
constantly being produced.

Whenever an organism respires or a fire burns, oxygen is re-
moved from the atmosphere and carbon dioxide is added to the
air. Decomposition of organic matter also liberates carbon dioxide,
and photosynthetic activity of plants removes carbon dioxide
and liberates oxygen. When these processes are not in balance,
there may be local variations in the composition of the air, but so
long as the air is not strictly quiet, the least motion, combined with
diffusion, is sufficient to eliminate gaseous differences almost at
once.

Thus, regardless of its terrestrial environment, the organism is
almost certain to be plentifully supplied with these gases that form
a relatively constant part of the atmosphere; therefore, these need
not be considered as variable factors in the environment.

GASES OF THE SOIL ATMOSPHERE

Although normally there is never a shortage of oxygen in the
air above ground, such a shortage sometimes occurs in the soil.
Air space in the soil is limited and is partially, or sometimes wholly,
occupied by water. Any change in the composition of the soil at-
mosphere is only slowly readjusted from the atmosphere above,
for here air movement and diffusion are relatively slow.

Since all living structures in the soil respire, and this includes
small animals and other microorganisms as well as roots of large
plants, the supply of oxygen is constantly reduced and carbon
dioxide is released. As a result, the soil atmosphere always contains
less oxygen and more carbon dioxide than the air above. Oxygen
decreases with depth, and carbon dioxide increases. In the soil un-
der closed stands of vegetation, carbon dioxide often equals 5
percent and has been found in much higher concentrations. The
constant use of oxygen and its extremely slow rate of diffusion
when soils are saturated soon result in oxygen deficiency. Tern-

CLIMATIC FACTORS : THE AIR 77

porary saturation may not be serious, but, when prolonged, it re-
sults in death of the vegetation through inhibition of root growth
and absorption. Under these conditions, several soil organisms may
carry on anaerobic respiration for a time, but such activity results
in chemical changes of several kinds, which may affect fertility of
the soil or actually inhibit plant growth.

Available oxygen in an aquatic habitat probably is somewhat
higher than in a saturated soil because of the movement of the
water and because the oxygen is more readily replaced by solution
from the atmosphere. If, however, the water is solidly frozen over,
it is not uncommon for the oxygen supply to fall so low that many
of the fish die. When such conditions develop in well-stocked
fishing lakes, it is now common practice to cut several holes
through the ice and to pump air through the water until the de-
pleted oxygen supply has been replaced. The mud at the bottom
of a shallow pond is probably the least favorable habitat for plant
roots. Most plants growing well in such places are of the emergent
type, having at least part of their structure in the air and charac-
terized by lacunar tissue, which permits gases to accumulate in,
and move freely through, the plant.

WATER CONTENT OF THE ATMOSPHERE

In addition to the gases constituting the atmosphere, water is
always present as vapor but in widely varying amounts. Since at-
mospheric moisture represents the indirect source of the plant's
water and likewise controls the amount and rate at which water
is lost by the plant, it is an environmental factor deserving some
attention.

The capacity of air to hold water vapor increases as tempera-
tures rise or pressure is reduced. The air is said to be saturated
when it contains as much moisture as it can hold at a given tem-
perature and pressure. If for any reason the temperature is raised
or the pressure is decreased, the amount of water remaining con-
stant, the air is no longer saturated. On the other hand, if the tem-
perature of saturated air decreases, the capacity is reduced, and
some of the vapor precipitates as a liquid. Thus air that is not sat-
urated will become so without change of vapor content if its tem-
perature is lowered, and, when saturation is reached, the air is said

78 THE STUDY OF PLANT COMMUNITIES • Chapter V

to be at the "dew point!' If the cooling continues, the vapor be-
comes a liquid, which may condense on objects near the surface
of the earth as dew or frost or, if condensation takes place in the
air, may result in precipitation.

Terminology of Atmospheric Moisture.— Several expressions
are used to describe the moisture content of the air. Absolute hu-
midity is commonly interpreted as the amount of water vapor per
unit volume of air and can be expressed as grams per cubic meter
or any other units of mass and volume. In itself the absolute hu-
midity has little bearing on the life of a plant, for it is not the total
atmospheric moisture that determines evaporation and transpira-
tion, but rather the difference between the weight of vapor pres-
ent and the maximum weight the air could hold at the time. Thus
the relative humidity, which is an expression of percentage of sat-
uration, is more nearly related to the rate of water loss from a free
water surface or from a plant. Relative humidity depends upon
temperature as well as the amount of moisture present, and, as a
consequence, identical relative humidities do not indicate identical
moisture conditions unless the temperatures are also the same. This
means that every shift in temperature results in a change in rela-
tive humidity, regardless of moisture present, and a consequent
change in rate of evaporation or transpiration.

Several authors have emphasized that, when considered inde-
pendently of other factors, the actual amount of water vapor in
the air has little if any influence upon evaporation. One illustra-
tion7 especially serves to emphasize the ecological significance of
this fact. Death Valley, California, is probably the most arid region
in the United States, yet its "dry" atmosphere contains on the aver-
age in July almost exactly the same amount of water vapor per
unit volume as does the "moist" atmosphere of Duluth, Minnesota,
at the same time of the year.

An atmosphere 70 percent saturated at 60° F. will contain much
less water vapor than an atmosphere 70 percent saturated at 80° E,
and the capacity to hold more water will be less in the first than
the second case. Evaporation will, therefore, normally be more
rapid at 80° E even though the relative humidities are the same. It
can be seen that a statement of relative humidity alone gives little
indication of atmospheric moisture conditions since a relative hu-

CLIMATIC FACTORS : THE AIR 79

midity of 80 percent may mean "dryness" if the temperature is
high or "wetness" if the temperature is low.

It is desirable then to have a term indicating the amount of water
that air can take up before it becomes saturated. Vapor pressure
is a measure of the quantity of water vapor present, the tempera-
ture being constant, and is usually expressed in units of pressure
(inches or mm. of Hg). Therefore, vapor pressure deficit is the
difference between the amount of water vapor actually present
and the amount that could exist without condensation at the same
temperature. It is a direct indication of atmospheric moisture,
quite independent of temperature and, therefore, compared to
relative humidity, its values are much more indicative of the po-
tential rate of evaporation.

When the relative humidity is 100 percent at 68° E, the vapor
pressure is 17.54 mm. of mercury. If the relative humidity were 70
percent, the vapor pressure would equal 12.28 mm. (0.70 x 17.54),
and the deficit would be 5.26 mm. (17.54-12.28). If the relative
humidity were the same (70%) at 59° E, the vapor pressure would
be 8.95 mm. (0.70 x 12.79) and the deficit would be only 3.84 mm.
(12.79—8.95). Tables of saturation pressures (vapor pressures) are
usually available in handbooks of chemistry, and it is possible to
transform relative humidities to vapor pressure deficits quickly
when the temperature is also known. The relationships are shown
in Table 6.

Greater general use of the vapor pressure deficit in ecological
work seems desirable, for in spite of certain limitations, its ac-
curacy is much greater than that of relative humidity. The poten-
tial rate of evaporation cannot be indicated with a single simple
expression of atmospheric moisture since the rate depends upon
the vapor pressure gradient between evaporating surface and at-
mosphere. The gradient can be determined only when the tem-
perature and vapor pressure of the liquid are known as well as
those of the atmosphere. Vapor pressure deficit is directly related
to evaporation only when the temperatures of the air and of the
evaporating surface are equal.252 Ecologists more often than not
measure evaporation directly, but when evaporation is not known,
in spite of the above, vapor pressure deficits could well be used in-
stead of relative humidities.

THE STUDY OF PLANT COMMUNITIES ■ Chapter V

Ftfpor pressure deficits (mm of Hg), reading down, at given relative humidities

• *-»

i-oooror- i oo r^- *-o t^- cm >-n »-o

100%

Vapor pressures (mm of Hg), reading up, at given relative humidities.

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CLIMATIC FACTORS : THE AIR 8 1

Measurement of Atmospheric Moisture.— The psychrometer
and the hygrometer are the two instruments most useful to ecol-
ogists for this purpose. The former consists of two thermometers,
one of which has the bulb wrapped with a wet piece of cloth, and
both of which are aerated in some fashion, usually by whirling.
Evaporation from the wet cloth is controlled by the moisture in
the atmosphere, and the bulb is cooled in proportion to the evap-
oration. The dry bulb gives the temperature of the atmosphere,

FlG. 31. A sling type of psychrometer for determining relative humidity
by the difference in temperature of the wet and dry bulb after whirling.—
Courtesy Friez Instrument Division, Bendix Aviation Corporation.

and the difference between the dry and wet bulb readings gives
the wet bulb depression. Knowing the barometric pressure and
these two values, the relative humidity can be quickly determined
from standard tables or from nomograms.108 The necessity for
aeration of the thermometers, usually accomplished by rapid ro-
tation, has led to the design of several "sling" type psychrometers.
Because these must be whirled, they require considerable space for
operation. The "cog" psychrometer, functioning like an egg beat-
er, can be used in much smaller spaces.

The hygrometer is usually a continuously recording instrument
in which an arm marks on a rotating drum the stretching and con-
tracting of a strand of hairs, which respond to relative humidity.
The drum is so calibrated that relative humidity is recorded di-
rectly. Often the device is equipped to record the temperature
simultaneously and is then called a hygrothermograph. Naturally,

THE STUDY OF PLANT COMMUNITIES • Chapter V

this automatic device is very convenient, particularly since it nor-
mally needs to be serviced but once a week. If, however, several
stations are to be maintained, the necessary instruments may not
be available, and the psychrometer is then the only solution.

With readings of the psychrometer and the hygrothermograph,
the air temperature is also obtained, providing the means of calcu-
lating vapor pressure deficits with no extra determinations. A

FlG. 32. A hygrothermograph, which automatically gives a continuous
record of relative humidity and temperature of the air.— Courtesy Friez In-
strument Division, Bendix Aviation Corporation.

simple nomogram (Fig. 33) permits direct conversion from wet
and dry bulb temperatures to vapor pressure deficit.

Evaporation and transpiration.— Measurement of transpiration
under natural conditions is often practically out of the question.
Although small plants may be potted or grown in cans and these
may be weighed at regular intervals to determine water loss, only
a limited number of plants can be used, and the labor involved can

CLIMATIC FACTORS : THE AIR 83

soon become prohibitive if a comprehensive study is to be made.

Relative rates of transpiration can be determined by the cobalt-
chloride method, which is rapid and permits numerous determina-
tions in a short time. Paper treated with cobalt-chloride is blue
when dry and turns pink as it takes up moisture. Small squares of
the dry paper can be attached to leaves between small glass plates
by means of a wire clip. The time required for the paper to turn
pink is taken as a basis of comparison. To get comparable values,
the paper must be absolutely dry and care must be taken that the
clip fits snuggly to the leaf. For increased accuracy, standard color
strips are usually attached to the glass to be used for comparison
with the sensitized strip. In spite of its simplicity, the method has
definite limitations. The close-fitting clips exclude all outside air
and thus eliminate air movement as a factor, while at the same
time diffusion into the air is practically stopped by the glass. Thus
the measurement is perhaps an indicator of the moisture in the in-
ternal atmosphere of the leaf. Rarely will two leaves on a plant
give identical readings, for their water loss varies with their posi-
tions on the plant and their ages. Thus several determinations must
be made simultaneously to evaluate a single plant, while to compare
this plant with others necessitates a considerable number of read-
ings. In spite of these limitations, it should not be assumed that the
method is ineffectual, for under certain conditions, it has been used
to great advantage.

These methods have their greatest utility in intensive studies of
a few or of individual plants under experimental conditions in the
laboratory or field. In studies of communities, it is often desirable
to have a more generalized picture of transpiration conditions.
Under those conditions, the rate of evaporation may be more use-
ful than a limited number of measurements of transpiration. Per-
haps the most desirable information is obtained by using plants as
instruments (phytometers). Two or more habitats may be com-
pared by setting up potted plants of the same species in each of
these habitats and comparing their transpiration rates as indicated
by loss of weight over the same period of time. Again the work
involved is often prohibitive. As a result, ecologists have largely
come to depend upon mechanical devices that measure evaporation
over unit periods of time, and, since evaporation and transpiration

84 THE STUDY OF PLANT COMMUNITIES • Chapter V

DRY BULB TEMP. V. R D. WET BULB TEMP.

(C°3 (mm. Hg.) (C°)

r~40 -,

- 35

- 30_

-20

-25_

-35

-2 5

-15-

_I0_

- 5 _I

L 0 J

-20_

-I 5_

- 10-

- 5 1

t-OJ

T-70

__60

_^50

__40

__30

_:l20

:_IO

-i. 0

r— 0 — i

- 5 -

- I 0-

-I 5_

-20_

-25

0 -i

- 5 -

-10-

-15-

-20-

30-

-35

.^40 J

Bo romet e r So romerer

FlG. 33. Nomogram for the direct conversion of wet and dry bulb tem-
peratures to vapor pressure deficit, at barometric pressures of 30, 29, 27, and
25 inches. To use, lay a straight edge across the appropriate temperature
values on the wet and dry bulb scales and read off VP.D. directly. Because of
the reduction necessary for this reproduction, extreme accuracy is not pos-
sible in its use— By permission W. E. Gordon.108

CLIMATIC FACTORS : THE AIR 85

respond similarly to the external factors affecting the latter, evap-
oration is taken as indicative of potential transpiration.

Evaporation is measured by the United States Weather Bureau
by means of large open tanks of uniform size and depth, but this
method is quite unsatisfactory for most ecological purposes. The
bulkiness of the equipment, the necessity for frequent checking,
and the probability of disturbance and of contamination are all
against it.

Various compact evaporimeters have been devised primarily for
ecological use. Of these the now well-known Livingston atmom-
eter has been most widely used. It consists of a porous clay sphere
or cup connected to a reservoir by means of a tube. Water evap-
orates from the clay surface and is constantly replaced from with-
in. If the sphere and tube have been filled with distilled water so
that no air bubbles are present (most easily done under water),
the water will be drawn from the reservoir through the tube. An
additional small-bored tube passed through the stopper of the res-
ervoir will permit equalizing of pressure but negligible loss of
water by evaporation. The reservoir is marked near the top and
filled to this mark by lifting the stopper. Subsequent fillings made
at regular intervals indicate water lost to the air by evaporation
over the period of time involved.

The simplicity of this device has been in its favor, and it has
other advantages. Before they are sold, all atmometer cups are
checked against a standard and marked with a correction factor
which, when applied, permits direct comparison of results obtained
with every instrument wherever it is used. If, as is frequently true,
the cups become dirty or accumulate a film of algae, they must be
restandardized. If algae and fungi tend to accumulate in the reser-
voir or on the cup, they can usually be controlled by a small piece
of copper sulfate in the water. The error produced by the solute
is negligible as compared to that caused by a film of algae.

The spherical form of the atmometer cup gives it the advantage
of exposing half its surface to the sun regardless of the sun's posi-
tion. Other evaporimeters with different shapes have been less
useful for this reason alone. Black cups can be used in combination
with white and the increased evaporation resulting from their
greater heat absorption may be used as a measure of relative light
intensity in different habitats.

THE STUDY OF PLANT COMMUNITIES ' Chapter V

Since the cup permits evaporation, it also will absorb water dur-
ing rainy spells. For field work, therefore, it is necessary to install
one of the various mercury traps designed to permit water to rise
in the tube but not to let it return to the reservoir. A simple but
effective trap consists of a drop of mercury in the lower end of
the tube, held in position between two plugs of pyrex glass wool.

t r?

FlG. 34. Two atmometers set up and in use in a study of grassland en-
vironment. The improvised shelter was used for max-min thermometers.—
Photo by R. B. Livingston.

When temperatures fall below freezing, atmometers cannot be
used because of the danger of breakage. A summary of the devel-
opment, uses and limitations of atmometers is given by Living-
ston.157

Condensation of Atmospheric Moisture.— If air is sufficiently
cooled, its relative humidity increases to 100 percent, or saturation.
The slightest cooling beyond this point will result in condensation
of the vapor to form a liquid. The temperature at which condensa-
tion occurs (dew point) will, of course, vary with the moisture
content of the air.

CLIMATIC FACTORS : THE AIR 87

Cooling of air masses is commonly caused by their expansion
when air rises in convection currents or when moving air is forced
to rise, as when it strikes a mountain slope. Cooling also occurs
cyclonically, for then masses of warm and cool air may meet, and,
depending upon which is least stable, warm air moves up over the
cool (a warm front) or the cold air underruns the warm (a cold
front). Of considerable local ecological significance is the contact
cooling resulting when relatively warm air moves over a cooler
surface or when cold air moves in over a body of warm water.
Under these conditions, fogs or clouds may form, which not only
may result in precipitation but may also modify the effects of
solar radiation.

Fog.— Any minute particles of matter in the atmosphere with
hygroscopic properties may serve as condensation nuclei (there is
disagreement as to their necessity) about which droplets of water
form, the size of the droplets depending upon the speed of con-
densation. Contact cooling usually produces only small droplets,
which remain in the air and are visible as fog. Coastal fogs are of
this type when they are the result of prevailing winds coming off
the warm ocean and striking a cooler land mass. Such fogs are
usually dissipated as the day progresses, evaporating as the tem-
perature rises. Coastal fogs may also be caused by winds blowing
from areas of warm water across cool currents. In summer, along
the Pacific coast, warm air moves in from far offshore across the
cool California current flowing from the north. Fog forms over
the cold current and is blown inland, where it disappears if the
land is warm but persists at night when the land is cooler. Because
they affect light, temperature, and moisture conditions, fogs may
be of extreme importance in determining types of coastal vegeta-
tion and the agricultural possibilities of an area. The distribution
of coastal redwoods of our Pacific coast forms a striking example
of the effects of fog. In a region almost without summer rainfall,
the coastal redwood and several associated species are almost pre-
cisely limited to the humid fog belt along the coast. Fogs inland
are usually over low ground, swamps, or small bodies of water,
and are common in valleys where air movement is reduced and
radiation cooling is effective.

Clouds.— Clouds differ from fog only in their position. Both are

88 THE STUDY OF PLANT COMMUNITIES • Chapter V

made up of droplets of water suspended in the air because they are
so minute that they do not settle out. Clouds are frequently formed
when air is carried upward by convectional currents and is cooled
to the dew point as it rises. Cooling and condensation with con-

FlG. 35. Ocean fog pouring over crest of Coast Range, Oregon.— Photo
by IV. S. Cooper.

sequent cloud formation also result when air is forced upward
over a mountain range and from cyclonic disturbances.

Clouds are classified on the basis of form and position, the termi-
nology being derived from an early simple classification in which
four types were recognized : cirrus (curly), cumulus (piled up),
stratus (flat), and nimbus (rain or storm). Modern systems divide
clouds into families, each with its own type of clouds distinguished
by descriptive names that are combinations of the old terminol-
ogy.265 For details about clouds and cloud forms, an illustrated
manual should be consulted.261' 128

Precipitatio72.—Fogs and clouds reduce intensity of solar radia-
tion that reaches the earth and may thus be of constant, though
minor, ecological significance in certain areas. But, of more gen-
eral importance, they are the source of precipitation when, be-
cause of rapid condensation, their tinv droplets increase in size
sufficiently to respond to gravity and fall to the earth. Not all
clouds produce rain because convection may not be rapid enough
or persistent enough to produce drops of sufficient size. Summer
rains are frequently short and heavy because of local, vertically
ascending air currents of high velocity. During cooler seasons, rain

CLIMATIC FACTORS : THE AIR

is more apt to result from the slow ascent of warm air currents
along atmospheric fronts or great shifting air masses. In the vicinity
of mountains the same effect is obtained by moist air being forced
upward to altitudes of lower temperature and density. The high-
est precipitation records are usually found on windward slopes of
mountains and are produced by such forced ascents of air. Tropi-
cal rains, although often very heavy, are usually convectional in
origin.

FlG. 36. Coastal redwood forest in California, showing the characteristic
morning fog that is a factor in its survival.— U. S. Forest Service.

THE STUDY OF PLANT COMMUNITIES ■ Chapter V

Other forms of precipitation include snow, which is formed
like rain but at temperatures below freezing and under conditions
that permit the crystals to fall before they melt. Sleet is rain that
falls through air strata of low temperature and then reaches the
earth as clear pellets of ice. If rain falls on a cold surface and
freezes, it is called glaze. Hail, which falls almost exclusively in

FlG. 37. Northern hardwood stand of birch, hard maple, elm, and ash
after a glaze storm in New York. Scarcely a tree escaped damage.— U. S. For-
est Service.

summer because of its dependence on convectional storms, starts
with a snow or ice nucleus, which falls to a stratum of sufficiently
high temperature to be partially melted. When carried upward
again, the moisture on the surface freezes, and condensation adds
to the size. If the process of falling and being carried up again
is repeated several times, the successive thawing, freezing, and con-
densation will form a concentrically layered mass of snow and ice
of sufficient size to fall to the earth as a hailstone.

Since hail is primarily a summer phenomenon occurring only
under exceptional conditions, it is of little consequence to plants
as a source of water. It may, however, do serious phvsical damage,
often stripping foliage completely from woody plants and damag-
ing herbaceous structures beyond recovery. Sleet and glaze are in

CLIMATIC FACTORS : THE AIR

the same category. Glaze may be so heavy as to cause great dam-
age to forest trees through breakage. Conifers are particularly sus-
ceptible to such damage because of the load of ice that can accu-
mulate on their many needles. In young stands, the trees may be
broken down so that they die, or they may be so bent and twisted
that, should they grow to maturity, they form badly distorted
trees.

FlG. 38. Average snow pack as it appears in March in the Sierra Nevada.
Echo Summit, Calif —Courtesy of W. D. Billings.

Snow is an important source of soil moisture and, in addition,
may serve to modify the effects of low temperatures. Roughly
ten inches of snow are equivalent to an inch of rain although the
moisture content of snow is highly variable. Under average tem-
perature conditions, water derived from melting snow might
make up from 5 to 25 percent or more of the total precipitation,
but its importance is not determined entirely by amount. Since
conditions in the spring may be such that a heavy blanket of
snow disappears in a few hours, the water may run off rapidly,

92 THE STUDY OF PLANT COMMUNITIES • Chapter V

especially if the soil is frozen, and be of no more significance
than that of an extremely heavy rain of short duration. That
same amount of snow, if it melts over a period of weeks, can re-
lease water so slowly that practically all of it will soak into the
soil, to become a part of a reservoir to be drawn upon during dry
periods weeks or months later. Again, under semidesert condi-
tions where the vapor pressure deficit is high, this may not be
true because, if the snow remains for long periods, much of it may
be lost by evaporation or sublimation.

2.00

1.60

O 1.20

H 0.80

0.40

'rainfall

(FIELD) '

1 1

C FOREST)

1 1

-t — p
1 \

1 '
1 -

\ RUNOFF

CFIELD)

RUNOFF f FOREST)

-i

i
i
t
n
ii
ii

ii
ii
u
• i

1 1

-i i

. 1 INFILTRATION CFOREST)

\ —

i /
i /

X-„INF_ILT RATION

_CFIELD)_

i /

* 1 1

1 ^s

60 80 100

MINUTES FROM START

120

140

FlG. 39. A comparison of surface runoff and infiltration on forested pine-
land (55 yr.) and on bare, abandoned land in Mississippi when precipitation
was at essentially the same rate for both areas.— Adapted from Sherman and
Musgrave.232

This reserve of ground water derived partly from snow be-
comes of greatest importance where the total precipitation is
relatively low. The grasslands of our Middle West are much
more dependent upon the reserve of ground water than are
forested regions where the total precipitation is greater and is
more evenly distributed throughout the year. The success of
agriculture, especially wheat production in the mixed prairie
region of the Dakotas, Nebraska, Colorado, and Wyoming, is, to
a great extent, dependent upon the reserve of soil water derived

CLIMATIC FACTORS : THE AIR

from snow. To be sure, where snowfall comprises a high per-
centage of the total precipitation, it must be of relatively greater
importance than elsewhere. Subalpine forests are often almost
completely dependent upon soil water derived from snow. The
red fir forest along the crest of the Sierra Nevada receives prac-
tically no rain throughout the growing season. However, the
cool summer days at these high altitudes do not create high water
losses, and since snow falls of sixty feet have been recorded here,
the resulting water is adequate to maintain the forest and to pro-
vide, as it runs off, an excess usable for agriculture at lower alti-
tudes.

FlG. 40. A standard rain gauge and measuring stick. Cutaway view to
show funnel and inner tube.— Courtesy Friez Instrument Division, Bendix
Aviation Corporation

94 THE STUDY OF PLANT COMMUNITIES ■ Chapter V

Snow water is of prime importance in those arctic and alpine
regions where there is practically no rain. Here the plants are
shallowly rooted, not uncommonly limited to the surface soil by
perpetual frost a few inches below. Surface water must then be
supplied continuously to maintain plant life. This is provided by
the melting snow, some of which, in depressions or other pro-

AVERAGE ANNUAL PRECIPITATION FOR THE UNITED STATES

EXPRESSED IN INCHES

75 10 100
and owr

FlG. 41. Average annual precipitation for the United States.— By permis-
sion, from Bernard,19 in Hydrology, copyrighted 1942, McGraw-Hill Book
Company.

tected places, may remain throughout the growing season. The
richest flora in best condition will usually be found at the margins
of snow patches and in drainage lines below them. Ridges and
raised ground are the first to be exposed at the beginning of the
growing season, and there growth begins almost immediately. As
the season progresses, more ground is exposed by melting snow,
and plants there begin growth. Thus, at distances of a relatively
few feet, may be found plants of the same species, that have
flowered, fruited, and dried up, and, in the moist soil beside the
snow, plants which have just begun their growth.

The total annual precipitation of an area is only a rough indi-
cation of moisture conditions for plant growth. A light rain of
0.15 inches usually does not affect soil moisture, for most of it
will be intercepted by vegetation and will evaporate quickly.
That which reaches the soil will wet only the surface and like-

CLIMATIC FACTORS : THE AIR

wise be lost to the air. Several inches of the total rainfall may,
therefore, be of no significance whatever except to raise the hu-
midity temporarily and reduce transpiration for a short time. If
rain falls heavily for short periods, say two or three inches in the
same number of hours, much of it will be lost by runoff, the
amount varying with steepness of slope, nature of the soil, and

Grassland Deciduous Forest Deciduous Forest Coniferous Forest

Cheyenne. Wyo. Indianapolis. Ino. Richmond.Va Ottawa, Canada

Jan.

JULY

FlG. 42. Annual precipitation patterns (based on averages) for several
stations, which illustrate the relative amounts and distribution of precipitation
throughout the year for areas supporting grassland, deciduous forest, and
coniferous forest.— A dapted jrom Transeau.2

256

amount and kind of cover. Again, the seasonal distribution of
rainfall may be of much more importance than the total amount.
If rainfall is uniformly distributed throughout the growing sea-
son, moisture conditions may be far more favorable with twenty-
five to thirty inches than they would be with forty to forty-five
inches if the growing season is interrupted by one or more pro-
tracted dry spells. If precipitation is regularly seasonal, the type
of vegetation may be definitely limited. For instance, grasslands
characterize those areas where rainfall is rather light and con-
centrated in the spring and early summer. Winter rains with dry
summers, characteristic of several coastal regions, support shrub-
by vegetation.

Measurement of Precipitation.— A standard rain gauge is a cyl-
inder 8 inches in diameter and 20 inches high, which has a funnel
built into the upper end that permits the water it catches to run
into an inner cylinder with exactly one-tenth the cross-sectional
area. The ratio of the outer to the inner cylinder being 10:1, the
measurement of water collected in the tube must be divided bv
ten if taken directly, or it can be measured with a standard

THE STUDY OF PLANT COMMUNITIES • Chapter V

graduated rod. The 10:1 ratio makes accurate readings possible
to 0.01 inch. Exceptionally heavy rains may overflow the tube,
and the water in the large cylinder must then be poured over
into the emptied tube for measurement. Two types of recording
gauges are in use.19 One registers increments of fall as a small
bucket fills, tips, and records; the other, a weighing type, records
accumulative precipitation as it falls.

Chestnut-Chestnut i
Longleof-

r^V^ Tall Grass
\/ / /\ Short Gross

E353 Bunch Grass

5223 Marsh Grass EZZgChOPOTTOl
WZZZZ Desert Savanna EZ22 Pacific

Sagebrush

Douglas Fir

[Xm Creosote Bush ^^ °^heT Western
ESS3 Greasewood Forests

K/H

Forests
Oak- Hickory

^^ Oak- Pine

FlG. 43. Isoclimatic lines of vapor pressure deficits and vegetation areas
of the United States.— From Huffaker.vlt

125

For generalized field studies, the precipitation records from the
nearest weather station may be quite satisfactory. However, there
may be wide local variations, especially if the topography is
irregular, and, in mountainous regions, only local measurements
have real significance. In addition, under forest stands, the pre-
cipitation reaching the soil will vary from stand to stand because
of variation in interception. Thus, for intensive work, it is desir-
able to maintain rain gauges at each site of study. Although stand-
ard gauges are desirable, it is possible to obtain satisfactory
records for comparison by using straight-walled jars or cans of
equal diameter.

Snowfall is measured at a point where the wind has not caused
drifting or disturbance, and the equivalent value in rain is com-
puted from samples of the snow. Depending upon the density of

CLIMATIC FACTORS : THE AIR

the snow, the ratio may range from 5:1 to 50:1, but 10:1 is fairly
average. Careful records of snowfall and water equivalents have
not been generally kept until recently. In the western mountains,
where melting snow may be the only source of water for distant
low country, such records make possible forecasting of floods
and, more particularly, the supply of water available for irriga-
tion.54

FlG. 44. Precipitation-evaporation ratios for the United States calculated
according to Transeau.255— By permission from Jenny, Factors of Soil Forma-
tion, copyrighted 1941, McGraw-Hill Book Co.

Atmospheric Moisture and Vegetation.— It should be clear that
any single atmospheric factor is insufficient in itself to explain the
distribution and survival of species or plant communities. Pre-
cipitation records are only suggestive, for they must be inter-
preted in terms of seasonal distribution, and they are not at all
indicative of soil moisture conditions or of the evaporating power
of the air to which a plant must be adjusted if it is to survive.
The variation in the seasonal pattern of precipitation from place
to place becomes particularly apparent when illustrated with
twelve-point polygonal diagrams,256 which make possible easy
comparison of amount and time of rainfall by months. Evapora-
tion alone is a poor criterion of ecological conditions since it does
not take into account the amount of water supplied to the soil.

THE STUDY OF PLANT COMMUNITIES • Chapter V

-30

^WINTER

-WINTER

DECIDUOUS EOREST.
BOREAL FOREST

1 •

90-

*'%

SUMMER N.

""I

I—

80-

~"\ /

\ '

70-

\ /

\ ,'

1—

60-

1 1 i' \

\ ' \

; I / \

I—

50-

• \ / \

IV / H

40-

» \ ' \

\ l y \

i V' \

v 1 )

30-

\ WINTER /

SO DESERT SHRUB....
SAI-.FRRIISW

20-f

1 2

1 r— - — i

3 4 5 6

MEAN MONTHLY RAINFALLOnche;

MEAN MONTHLY RAINFALL(inches)

'. SUMMER

WINTER

SHORT GRASS.
TALL GRASS-

MEAN MONTHLY RAINFALL(inches)

90-

80-

UJ 70-
Q_

40-

30-

SUMMER

WINTER

ALBUQUERQUE. N.M.

EL PASO, TEXAS

2.5

MEAN MONTHLY RAINFALL (Inches)

Fig. 45. Three sets of composite climographs, which permit comparison
of forest, desert, and grassland climates, as well as differences within these
general types of vegetation. The fourth set, which shows the similarity of
climates at stations in New Mexico and Texas, has been used as support for
classifying El Paso grassland as short grass, of which Albuquerque is repre-
sentative, and not desert grassland as some have done.— From Smith™

When .points of equal vapor pressure deficits are connected by
lines on a vegetation map,125 the zones come nearer to matching
the distribution of vegetation tvpes on a regional basis than simi-
lar ones based on evaporation. Seeking a single comprehensive

CLIMATIC FACTORS : THE AIR

value that would include several factors operative in plant dis-
tribution, Transeau255 used the ratio of precipitation to evapo-
ration (P/E) for plotting climatic zones. These zones match the
limits of vegetation types remarkably well, but the method is
limited by the availability of adequate and comparable evapora-
tion data.

A graphic method for distinguishing differences and similari-
ties in atmospheric conditions is the climograph, in which mean
temperature is plotted against mean relative humidity by months,
and the points are connected to form highly distinctive twelve-
pointed figures. Introduced by Ball11 for indicating climate of

FlG. 46. Cup anemometer, Weather Bureau type, for relatively permanent
operation, and a Biram type anemometer, convenient for short-time measure-
ments.— Courtesy Friez Instrument Division, Bendix Aviation Corporation.

100 THE STUDY OF PLANT COMMUNITIES • Chapter V

geographic areas, it has been variously used for comparing cli-
mates in studies of the distribution, migration, and success of pop-
ulations of man, birds, and insects. The system is subject to modi-
fication and has been used also as a graph of temperature-precipi-
tation (sometimes called a hythergraph). The latter method has
been used243 for characterizing climates of widely differing climax
types in different parts of the world and for distinguishing grass-
land climates in North America.52 The method probably has not
been given the use it deserves in plant studies. Because tempera-
ture-relative humidity diagrams have been used with some suc-
cess, it seems reasonable to suggest that similar graphs of tempera-
ture-vapor pressure deficit might give even more distinctive pat-
terns and might, therefore, be even more useful in detailed studies.

WIND

Air moves from a region of high pressure to one of low pressure,
and the differences in pressure are largely the result of unequal
heating of the atmosphere. The equatorial regions receive more
heat than regions to the north or south; consequently, low pres-
sures normally exist in the lower latitudes. The tendency, then, is
for air to move from the poles toward the Equator, there to rise
and return toward the poles. This pattern, although true in gen-
eral, is modified by the deflecting action of the earth's rotation and
by differences in temperature resulting from oceans and land
masses.

Continents in temperate zones tend to become very hot in sum-
mer, and the resulting low pressures produce winds that blow
inland. The cold of winter reverses the pressure, and winds tend
to be outblowing. In mountainous areas or along sea coasts these
seasonal trends may have daily variations again produced by tem-
perature-pressure differences. Mountain valleys and slopes, which
are often warmed rapidly during the day, produce valley breezes
blowing upward. At night, the rapid cooling of bare high ridges
results in a flow of cold air down the valleys. The contrast between
day and night temperatures of land and water results in an off-
shore breeze at night as the land cools rapidly and higher pressures
result. During the day, the land again heats up rapidly above the
temperature of the sea, and an inshore breeze develops that may

CLIMATIC FACTORS : THE AIR 101

be noticeable for several miles inland. This brief summary of fac-
tors producing wind should serve to emphasize that air is almost
constantly in motion and should suggest that, within limits, the
general plan of motion is predictable for seasons and parts of the
earth.

Measurement of Wind.— Wind velocity is measured with some
form of anemometer. The cup anemometer used by the United
States Weather Bureau has three or four hemispherical or conical
cups, each attached to horizontal arms that rotate on a vertical axis
and thus drive a gear system, which turns indicator dials. These
are readable in miles per unit of time, usually expressed as miles
per hour. More elaborate instruments may be equipped with auto-
matic recording devices.

The cup anemometer is inconvenient to carry and operate in
the field. In the Biram portable anemometer, a small fan drives the
dial indicating air movement. The device is useful in small spaces
and for short readings. Since it has no vane, it must be set to face
the wind.

Physiological-Anatomical Effects of Wind.— The movement of
air being in general characteristic of all environments, plants are
largely unaffected by it under average conditions. In certain situa-
tions, however, wind may be an extremely important factor. Plants
growing in habitats exposed to continuous winds of moderate
velocity transpire more rapidly than unexposed individuals. If the
prevailing winds are from one direction, the side of a plant toward
the wind may be so desiccated that new growth is killed before it
is well begun. Lateral buds taking over the growth may or may
not survive, and a scrubby, matted growth develops on the wind-
ward side. To leeward, the new shoots are protected by the rest of
the plant, and growth goes on there, resulting, over a period of
years, in asymmetric growth forms of amazing shape. Such one-
sided growth is commonly found in exposed places at high alti-
tudes in the mountains where otherwise upright plants may be
prostrate and form mats fitting into hollows or behind protecting
rocks. Not uncommonly a forest stand on the protected side of a
ridge or in a ravine may appear as though every tree had had its
tip sheared to an exact height limit. Again, this is due to the desic-
cating effect of the prevailing wind.

102 THE STUDY OF PLANT COMMUNITIES • Chapter V

Asymmetric growth, matted vegetation, and sheared tops as
seen along the coast are likewise produced by wind to some extent,
but here an added factor plays a part. The wind picks up spray as

FlG. 47. Prostrate and matted, wind-sheared trees {Firms albicaulis, Tsuga
mertensiana) on a leeward slope near timber line, Mt. Hood, Ore. The
twisted form is commonly termed Krummholz.—U . S. Forest Service.

it comes in over the breaking waves. The spray may be carried
several miles inland, especially in severe storms, but its major
effects are most noticeable near the coast. The spray that strikes
any obstacle is dropped there and, of course, the salt from the
spray accumulates on that object. Few dune and coastal plants are
completely tolerant to salt spray, but, fortunately, most strong
winds are accompanied by rain, which minimizes the effects by
dilution and washing. If a severe windstorm is not accompanied
by or soon followed by rain, much vegetation will be injured or
killed by salt spray even for some distance inland.

Those plants growing near the beach are sprayed lightly almost
daily and, as might be expected, show different degrees of toler-
ance. This results in zonation of vegetation associated with expos-
ure to the wind.188 Undoubtedly salt spray is one of the strong
factors in determining the make-up and distribution of all plant
communities on coastal dunes.270

When trees grow on one side only, they may become so heavy
as to uproot themselves, but usually the eccentric growth is slow
enough to permit compensating anatomical changes, particularly
in the trunk. Secondary growth may cease completely on the

CLIMATIC FACTORS : THE AIR 103

windward side of the trunk and increase proportionately on the
leeward side, thus forming a brace under the added top. An ex-
treme illustration is a section of trunk taken* from a Monterey
cypress that grew on Cypress Point, just south of Carmel Bay,
California. It is 74 inches in the diameter that grew parallel to the
prevailing wind but is only 9 inches in the opposite diameter. Only
50 growth rings were formed on the windward portion of the
section, but the leeward portion (71 in.) has 304 rings. This section
was taken 24 feet above the ground.

Other physiological effects might be mentioned, but they are
largely brought about within the plants themselves through adap-
tations that serve to reduce the rate of transpiration through their
effects on stomata. In the drier sections of the country, such as the
plains and desert, the almost continuous dry winds increase tran-
spiration rates materially and serve to accentuate the effects of low
water supply.

Physical Effects on Plants.— Most people have seen the effects of
a strong wind (25-38 miles per hour) upon vegetation. It is not
uncommon for dead branches to be torn from trees; an occasional
tree, especially if overmature and diseased, may be blown down.

(—■■■■'■

FlG. 48. Asymmetric growth of a live oak (Quercus virginiana) exposed
to ocean wind and salt spray from the right. North Carolina coast.— U. S.
Forest Service.

*Collected by and in the possession of W S. Cooper, University of Minne-
sota.

104 THE STUDY OF PLANT COMMUNITIES • Chapter V

FlG. 49. A white pine stand in New Hampshire after the storm of 1938.
Such damage was prevalent over much of New England at the time- U. S.
Forest Service.

Closed forest stands usually suffer no major damage because the
trees give support to each other. With greater velocities the wind
becomes increasingly destructive. At gale velocities (39-54 m.p.h.)
branches are broken, and a full gale uproots trees with ease.

Many of the destructive storms along the Gulf coast approach
hurricane speeds, and it is fortunate that they infrequently reach
the mainland. The most destructive hurricane in recent years
(1938) moved northward along the Atlantic coast and struck in-
land at 70 miles per hour at Long Island and into west-central
New England. The destruction in its path was extreme. Whole
forests fell before the wind, the trees uprooted or broken off. An
added factor in the destructiveness of this storm was the saturated
soil, produced by a preceding period of heavy rain, which con-

CLIMATIC FACTORS : THE AIR

105

tributed to the ease and amount of uprooting and wind throw.
Storms of such force and destructiveness are rare in North Amer-
ica, but lesser winds may cause considerable damage. When closed
forest stands are thinned or selectively cut, the remaining trees are
subject to wind throw for a number of years even though wind
does not blow with great velocity.

In addition to physiologically-produced flag forms of woody
vegetation, there are those resulting from purely physical effects
of wind. A study of asymmetric trees in the Columbia River
Gorge152 showed that, when branches are continually bent in one
direction by prevailing winds, the branches become "wind trained"
and hold their positions permanently. Some grew completely
around the trunk from the windward to the leeward side. Still
another cause of asymmetry was found here. Severe winter storms,
coming largely from one direction, cause much breakage, espe-
cially when accompanied by sleet, and almost complete pruning of
branches on the windward side often results.

Transportation by Wind.— We have already indicated how im-
portant to precipitation are the vapor-laden winds moving inland

FlG. 50. Wind throw often results when trees are uprooted, especially if
on shallow or wet soil. Here is shown a giant Douglas fir in Washington
whose torn-up root system had a spread of fifty feet— U. S. Forest Service.

106 THE STUDY OF PLANT COMMUNITIES • Chapter V

FlG. 51. Flag-form trees in the Columbia River Gorge, Ore. (1-2) Storm-
pruned Douglas fir, deformed by breakage and killing due to glaze storms
and strong west winds of midwinter. (3-4) Wind-trained Douglas fir shaped
by long-continued pressure of strong west winds of late spring and summer.—
From Lawrence.1'32

from large bodies of water and how transporting salt spray may
be of local significance. Wind plays a more direct role in trans-
porting pollen and in dissemination.

CLIMATIC FACTORS : THE AIR 107

Wind-borne Pollen.— Many pollen grains are light and small or,
as in conifers, have bladder-like wings, which increase their buoy-
ancy. As a result, they may be carried for great distances by the
wind. The chances that an individual pollen grain will accomplish
its function must be extremely small. This uncertainty is compen-
sated for in quantity of pollen produced. Efdtman97 gives the
pollen production for several wind-pollinated European species
from which the following are selected : Rumex cicetosct produces
30,000 grains per stamen, Acer platanoides, 1,000; pollen output per
staminate cone of gymnosperms may be judged by Pinus nigra,
1,480,000; Ficea excelsa, 590,000; and Jwiiperns communis, 400,-
000; production per flower of angiosperms ranges from Rumex
acetosa, 180,000, through Tilia cordata, 43,500, to Acer platan-
oides, 8,000. Such figures for single stamens and flowers serve to
explain the continuous and enormous rain of conspicuous pollen
that may fall in season, especially from conifers. Sidewalks,
porches, floors, tables— everything in the vicinity of a coniferous
forest— may be dusted with pollen.

Not all noticeable pollen is locally produced, and a great deal of
evidence has been accumulated to show irregular and normal dis-
tributions. There is a story that, in the early days of the city of St,
Louis, it was at one time continuously showered with a yellow
dust, which gave residents some concern until botanists identified
it as pollen of Pinus palustris transported from the coastal plain far
to the south. Some quirk of pressure and wind was depositing the
pollen upon St. Louis.

There are numerous records of pollen being transported long
distances.97 Spruce, pine, and birch pollen was collected on light-
ships in the Gulf of Bothnia thirty and fifty-five kilometers off the
coast. Spruce pollen is carried from southern to northern Sweden.
Peat samples taken in Greenland contained pollen of Picea mari-
ana and Pinus banksiana, which must have originated on Labrador
or southwestward. One of the most interesting studies of pollen
transport was made by Erdtman as he crossed the Atlantic from
Gothenburg to New York. Using a vacuum cleaner equipped with
filters, he obtained a more or less continuous quantitative record
of pollen in the air on the entire trip. Numbers of grains decreased
with distance from land, but at no time did sampling fail to show

108 THE STUDY OF PLANT COMMUNITIES • Chapter V

some pollen. The evidence is to the effect that birch, pine, oak,
willow, sedge, and grass pollen are carried in quantities for more
than one thousand kilometers over the ocean.

The amount of pollen in the air and the distance it is trans-
ported is of significance to some plants but more so to many
people who suffer from hay fever. Recently the kinds and num-
bers of pollen grains in the air in many sections of the country
are determined daily and made publicly available for the use of
hay-fever sufferers.

In general, wind-pollinated plants grow in the open or in ex-
posed places. Even in a forest it is the trees of the upper strata that
are characteristically wind pollinated; the flowers are small and
inconspicuous, with simple or reduced structure. The corolla is
often lacking, and there is an absence of bright colors, odor, and
nectar. Stamens and pistils are commonly borne in different flow-
ers, the stigmas are usually feathery, and the stamens are long and
pendant. In spite of its apparent wastefulness, the system produces
satisfactory results.

Dissemination— Plants migrate from one point to another by
means of spores, seeds, fruits, fragments of plants, or entire plants.
The agent of transport may be water, animals, or wind, depending
upon the various adaptations of the disseminules, which facilitate
the movement.

Dissemination by spores is characteristic of all plants except
spermatophytes. Wind-disseminated spores, like pollen, are small
and dry and may be transported great distances. Everywhere that
pollen is carried, spores are found too. Their transportation over
long distances can be of great ecological and economic impor-
tance. A spore carried by a freak wind into distant territory may
establish a species where it has never grown before, thus extending
the range of the species and possibly necessitating adjustments
within the community in which it develops.275 The economic con-
siderations are fairly obvious when it is remembered that fungi
that produce diseases of both plants and animals are all propagated
by spores. The fight against wheat rust is a case in point. When-
ever a resistant strain of wheat is developed, it is immediately sub-
ject to attack by mutating strains of the rust, whether these strains
are of local origin or not. There is evidence that strains of rust

CLIMATIC FACTORS : THE AIR

109

appearing in the Dakotas have come from wind-borne spores pro-
duced as far away as Mexico.246

Seeds, fruits, and fragments of plants are effective -as dissem-
inules in proportion to the devices that facilitate their transport.
Wind dissemination is increased by the presence of winged struc-
tures, bladder-like protrusions, or plumose extensions of the

FlG. 52. Approaching dust storm near Springfield, Colo. (1937), which
was typical of conditions in the "dust bowl" during the drought of the 1930's.
— U. S. Forest Service.

surface (see Fig. 93). Seeds, because of their small size, are apt to
be carried farther than fruits, but for all, the kind of adaptation is
an important factor in transport. The perfection of the parachute-
like pappus is illustrated by the ubiquitous dandelion and related
composites of field and roadside. Many winged fruits do not travel
far because of their size, but often the wings (ash, elm, maple,
basswood) are sufficient to assure transport beyond the shading
and competitive effects of the parent tree.

The transport of entire plants is well illustrated by the tumble-
weeds (Salsola, Cycloloma). These have but a single main root,
which, when broken at the ground surface, releases the spherical
plant to roll before the wind until caught, perhaps in some fence
corner. As it rolls, the seeds are gradually shed, sometimes miles
from the place of growth.

The pioneers in a new habitat usually have effective means of
dissemination and an abundance of seed. The same is true of weeds
of cultivated fields and waste ground. The more common and

110 THE STUDY OF PLANT COMMUNITIES • Chapter V

FlG. 53. Soil blowing out of a Kansas wheat field in the 1930's and piling
up on highway, where fences and trees partially checked its movement.—
U. S. Forest Service.

FlG. 54. Road cut through a deep deposit of loess in iMissouri. The al-
most vertical banks have stood for eighteen years without eroding.— U. S.
Forest Service.

CLIMATIC FACTORS : THE AIR

111

FlG. 55. Extensive active sand dunes on the coast of Oregon showing
transverse ridges that have typical form with gradual slope to windward and
an abrupt drop to leeward.— Photo by W. S. Cooper.

FlG. 56. Blowout in Oregon coastal dune that was once completely stab-
ilized by vegetation. This is a compound blowout as indicated by the partial-
ly stabilized surface of an earlier blowout (lower right), which was again
excavated to a lower level by later blowout.— Photo by W. S. Cooper.

112 THE STUDY OF PLANT COMMUNITIES ■ Chapter V

widespread a species is, the more efficient are the mechanisms that
facilitate its dispersal, regardless of whether the agent be wind,
animals, water, ice, or gravity.

Wind and Soil— The, slightest air movement shifts dust particles
from place to place, and increasing velocity results in the transport

^^^^l^m^r^^^

(fa£j.

FlG. 57. "Graveyard forest" near Florence, Oregon. Once a closed stand
(probably mostly Finns contorta) growing on the soil layer which is broken
through in the foreground, the forest was completely buried by the dune
now appearing in the rear which subsequently moved on to uncover it again.
The view is to leeward.— Photo by W. S. Cooper.

of larger particles of soil in increasing amounts. Although fine
materials are everywhere being shifted by wind, its greatest effects
are noticeable in dry climates where there is a prevailing wind and
a minimum of vegetation. During extended droughts, the culti-
vated, semiarid regions of our Midwest and Southwest have at
times become shifting seas of drifting soil, and the clouds of fine
materials carried about in the air have given rise to the term, "dust
bowl!'

Over an extended period of time great quantities of materials
may be transported and deposited by wind, as is demonstrated by
the enormous deposits of loess in various parts of the world. This

CLIMATIC FACTORS : THE AIR

113

fine-grained, fertile soil occurs in deposits from a few to fifty feet
deep or more over thousands of square miles in the central Missis-
sippi Valley region. Our richest farm lands in Iowa, Nebraska, and
Kansas are on loess soils. The deposits occur along the Rhine in
Europe and in the pampas of Argentina, and reach their greatest
extent in Asia, particularly in north-central China. Loess probably
originated during the glacial period as dust was swept up from

Fig. 58. Coastal sand dune moving inland and encroaching on evergreen
maritime forest near Kitty Hawk, N. C. Grasses in foreground have been
planted.— Photo by C. E Korstian.

the barren flood plains of glacial rivers and carried high into the
air, from which it settled more or less uniformly over wide areas.
Sand beaches and desert regions are commonly dry, free of
vegetation, and swept by prevailing winds, which carry the soil
along near the earth's surface. Any obstacle that checks the veloc-
ity of the wind causes some of its load to be deposited and starts
a mound or ridge called a dune. Some dunes grow, by the deposit
of more sand, to a height of several hundred feet, but usually they
are much smaller. Most of the sand is deposited near the crest or
on the lee slope; this results in a characteristic gentle windward
slope and a sharp drop on the lee slope, the steepness of which is

114 THE STUDY OF PLANT COMMUNITIES * Chapter V

determined by the angle of rest of the sand. Because wind fre-
quently changes direction, dunes are rarely stable for long and
present a constantly shifting pattern. Along sea coasts they tend
to move inland as sand is carried from the windward side and
dropped down the lee side.

^aW***^''*

&*■<

FlG. 59. Planted grasses and brush fences set up on shifting sand as part
of a dune-stabilization program developed by the Civilian Conservation
Corps. The attempt was partially successful but was not followed up with
later work, which would have added to its success.— Photo by C. R Korstian.

A dune is never completely stable unless covered with a con-
tinuous mat of vegetation. Should this mat be broken for any rea-
son, a "blowout" results, which may enlarge and start again the
shifting of the entire dune. Many cottage owners have learned this
to their sorrow when— as has happened on Lake Michigan dunes—
they have returned after a single year to find their summer homes
almost completely buried under a shifting dune that had been stable
for years. The encroachment of dunes on forest areas is not un-

CLIMATIC FACTORS : THE AIR 1 1 5

common. Whole forest stands may be buried and subsequently,
with shifting winds, be uncovered again to expose "graveyard"
forests of dead trunks and branches.

The extensive dunes on the banks along the coast of the Caro-
linas have in recent years become increasingly active because their
cover was broken or reduced by overgrazing and other disturb-
ances by man. Acres of maritime forest have been buried, build-
ings have been destroyed, and channels in the waterways have
been blocked. Here, as in the dust bowl, are problems that require
drastic measures for solution, but such measures must take into
consideration the ecological factors involved. Cover crops, strip
cropping, mulching, and other modified methods of cultivation
are now general practice in the dust bowl and promise to give
some relief should an extended drought occur again. Long ago
many European coastal dunes were planted with forests and ef-
fectively stabilized. The Carolina dunes, though, occupy thousands
of acres with almost bare sand on which forests cannot be planted
until some stability is attained. Kill Devil Hill, the dune from
which the Wright brothers made their historic first flight, was
stabilized with grasses by Army engineers, after much effort and
considerable cost, through use of sodding, seeding, and watering.
Such methods are impractical on thousands of acres. The efforts
of the Civilian Conservation Corps were at least partially success-
ful. Taking only the native dune grasses, they transplanted them
according to several spacing systems and with some regard to
habitat variation over several hundreds of acres. Combined with
plantings, brush fences were installed at regular intervals across
the largest blowouts. A considerable part of their work has proven
effective.

GENERAL REFERENCES

R. F. DAUBENMIRE. Plants and Environment. New York : John Wiley and

Sons Co., 1948. 424 pp.
W J. Humphreys. Fogs and Clouds.
O. E. MEINZER. (ed.) Hydrology. New York : McGraw-Hill Book Co.,

1942. 712 pp.

C. W THORNTHWAITE. Atmospheric Moisture in Relation to Ecological

Problems.
G. T Trewartha. An Introduction to Weather and Climate.
H. B. WARD and W E. POWERS. Weather and Climate.

CHAPTER VI

CLIMATIC FACTORS: RADIANT ENERGY
TEMPERATURE AND LIGHT

The sun is the source of the earth's radiant energy (insolation).
This energy, radiating as waves, includes those wave lengths of
the visible spectrum that we term "light" and those that lie just
beyond the visible spectrum, called "heat" if slightly longer, or
"ultraviolet light" if shorter. The amount of insolation reaching
the earth is always reduced because of absorption by the atmos-
phere (6-8 percent), and as much as 40 percent may be reflected
by clouds. The remainder reaching soil or water on the earth may
be further varied by such factors as distance from the sun at dif-
ferent seasons, duration of radiation, and the angle of the rays with
the earth's surface. The last determines the amount of air through
which the rays pass, modifies the amount of reflection and absorp-
tion, and likewise controls the amount of energy falling on a unit
area simply by spreading or concentrating a given amount of
energy over more or less space. With these things in mind, insola-
tional variation with latitude and topography are more easily ex-
plained.

Insolation varies only slightly at the Equator because the angle
of the sun's rays never exceeds 231/2° from zenith, and the days
are uniformly twelve hours long. Twice a year, on March 2 1 and
September 21, called the equinoxes, the sun is at zenith at the
Equator at noon and its circle of illumination exactly reaches the
North and South poles simultaneously. After March 21, because
of the tilt of the earth's axis, the North Pole comes progressively
nearer to the sun until June 21, after which the shift is reversed
to bring the pole back to the equinox position by September 21.
The North Pole's movement away from the sun continues until
December 21, after which it starts its shift back to the June posi-
tion again. The shifting of the pole toward the sun causes the
circle of illumination to extend far beyond the pole and results in
continuous insolation at the pole during the June solstice, but,

116

CLIMATIC FACTORS : RADIANT ENERGY

117

since the December solstice results in diametrically opposite con-
ditions, it represents a period without insolation. Conditions in the
Southern Hemisphere are, of course, always exactly reversed.
Thus, because of differences in insolation, we have seasons

VERNAL EQUINOX
MAR. 21

SUN

WINTER

SOLSTICE

DEC.2I

AUTUMNAL EQUINOX
SEPT 23

Fig. 60. Diagrammatic representation of the changing position of the earth
with respect to the sun and its relationship to insolation and change of sea-
sons in the Northern Hemisphere.— Adapted from Trewartha.

258

marked by variation in length of day and temperature. Since the
periodic differences in insolation become more marked with dis-
tance from the Equator, the seasons likewise become more distinct
with increasing latitude. The greatest total insolation, however,
occurs at the Equator and decreases with distance from the Equa-
tor in spite of the increasing length of day. Toward the poles, in-
tensity of insolation is reduced because of the increasing angle of
incidence.

118 THE STUDY OF PLANT COMMUNITIES ' Chapter VI

These introductory statements refer to insolation as a whole. We
may now more conveniently consider separately the visible portion
of the spectrum known as light, and those longer, invisible wave
lengths known as heat, whose presence or absence are expressed
as temperature.

FlG. 61. Circle of illumination, areas of daylight and darkness, angles of
sun's rays at different latitudes, and differences in areas affected and thickness
of atmosphere penetrated at time of summer solstice.— Adapted from Ward
and Powers.265

TEMPERATURE

General Plant Relationships.— Each living thing is restricted to
a definite temperature range, which may be quite dissimilar for
different species and, depending largely upon the amount of water
in the protoplasm, may vary for individuals of a species. The wide
range of tolerance among species is illustrated, on the one hand,
by subarctic conifer forests where -80° F. has been recorded and,
on the other, by desert plants that withstand temperatures of 130-
140° F. Dormant structures such as seeds and spores are practically
without water and can, therefore, withstand the widest tempera-
ture variations and extremes.

Plant injuries from temperature changes are most often the
result of freezing, which desiccates the tissues when the pure water

CLIMATIC FACTORS : RADIANT ENERGY 119

on the cell walls crystallizes in the intercellular spaces and con-
tinues to crystalize as it is replaced from the vacuole and proto-
plasm. Injurious chemical changes, such as the precipitation of
proteins, may accompany the desiccation. Some species, however—
especially subtropical ones— are often killed before temperatures
fall as low as freezing. Temperature injuries cannot always be ex-
plained in simple terms.

It is obvious that there must be seasonal and other adjustments
in some plants, which permit their survival as cold weather comes
on. It is known, in this connection, that the concentration of the
cell sap of most conifers increases in the fall. Gardeners make use
of this characteristic, for young plants grown in greenhouses are
"hardened" before they are set out and subjected to early spring
temperature fluctuations. Such plants are most liable to injury
when temperature changes are abrupt and extreme. On the other
hand, many arctic and alpine species can grow, flower and fruit
during a period when they are subjected almost daily to alternate
freezing and thawing.

Measurement of Temperature.— Accurate standardized glass
thermometers are the most useful instruments for field studies. Air
temperatures are usually taken in the shade with the thermometer
exposed to the wind and away from the influence of one's body.
Soil temperatures require a small well of some sort, or, when meas-
urements are to be made periodically, a length of pipe may be
permanently sunk to the desired depth. If the thermometer is
suspended in the pipe by a string, it can be drawn up quickly and
read before much change takes place. Soil temperatures at or very
near the soil surface are difficult to obtain accurately with an ordi-
nary thermometer because of the steep gradients from the surface
downward, and upward into the air. The size of the thermometer
bulb is sufficient to be affected by rather widely differing tempera-
tures even when it is no thicker than 5 mm. Discrepancies have
been observed as great as 1 1 ° C. between electrical (thermocouple)
and ordinary thermometer readings at the surface. The errors are
greatest in full sunlight and on dark soils.86 It is under these con-
ditions that the greatest care must be taken in placing the bulb.

Continuous temperature records are obtainable with thermo-
graphs. These usually consist of an expansion element attached by

120 THE STUDY OF PLANT COMMUNITIES * Chapter VI

levers to a pen, which records on a graduated sheet revolving on a
drum. Both air and soil thermographs are used and are also obtain-
able in the same instrument, thus giving parallel records. If cost of
these instruments prohibits their use where a comparative study
of numerous stations is to be made, maximum and minimum ther-
mometers are the simplest solution. Placed in pairs and read and

FIG. 62. Soil-air thermograph, which records the temperatures of soil and
air continuously on a revolving drum. The cable at right is about six feet
long and terminates in a sensitive bulb (not shown), which can be placed at
any level in the soil— Courtesy Friez Instrument Division, Bendix Aviation
Corporation.

reset at regular intervals, they give the useful values of maximum
and minimum temperatures for the period of exposure. Above the
reservoir in a maximum thermometer is a constriction through
which, because of the force and volume of mercury involved, ex-
pansion easily forces the liquid. Contraction, however, develops
no pressure above the constriction, and the capillary column re-
mains essentially at the level of its highest rise. As with a clinical
thermometer, the column must be shaken or spun back down to
the reservoir when a new reading is desired. The minimum ther-
mometer has a small marker in its liquid, which, because of surface
tension at the top of the column, is pulled down as the tempera-
ture is lowered but is not raised with increasing temperature. Tilt-
ing the thermometer will immediately bring the marker back to
the top of the column in a new setting position.

CLIMATIC FACTORS : RADIANT ENERGY

121

Temperature Records.— Because temperature is so extremely
variable, isolated or even numerous single determinations may be

Fig. 63. Maximum-minimum thermometers of a standard type for air tem-
peratures. Installed in an instrument shelter, the holder permits whirling of
the maximum thermometer for resetting.— Courtesy Friez Instrument Divi-
sion, Bendix Aviation Corporation.

completely useless. A continuous record is most desirable because
it gives the duration of extremes and variations. Although extremes
may be important in the reaction of a plant, their duration is apt
to be what determines the plant's response. Therefore, a thermo-
graph is desirable for thoroughly satisfactory work. The "mean
temperature" as computed by the United States Weather Bureau141
is usually the average of the maximum and minimum for the day.
This is not accurate or truly indicative of plant-temperature rela-
tions because it ignores duration and is likely to run too high. The

122 THE STUDY OF PLANT COMMUNITIES ' Chapter VI

true mean is more nearly approached by averaging the hourly
temperatures for twenty-four-hour periods.

Annual mean temperatures are almost useless ecologically, for
they do not indicate seasonal variation and duration. Temperate
desert regions may have amazingly high annual mean temperatures
and yet have winter frosts, which constitute an important limiting
factor in the survival of certain species there. Subarctic areas may
support forest vegetation because of the warm summers, yet mean
temperatures may be so far below freezing that they suggest that
little if any plant life would survive. It can be seen that mean
monthly temperatures are desirable for evaluating ecological con-
ditions, and this is equally true for monthly mean maximum and
minimum values. Collectively, these indicate the extent of the
growing season and the extremes to be expected during that time.

Temperature Variations.— Since fluctuations of insolation result
in fluctuations of temperature, seasonal and daily temperature
changes, as with insolation, can be expected to follow a general
pattern for any region. The pattern follows that of insolation but
with temperature responses lagging behind changes in radiation.
A daily maximum of atmospheric temperatures usually comes in
midafternoon, and minimum temperatures occur just before sun-
rise. Soil temperatures lag even more, for their maxima may not
occur until 8:00-11:00 P.M. and minima may not be reached until
8:00-10:00 A.M. This is, of course, due to the fact that soil is a
poor conductor of heat. For the same reason, the soil surface, if
unshaded, produces the highest temperatures for an area and like-
wise has the widest range of temperatures. It is the subsoil tem-
perature that follows the trend indicated above. With increasing
depth, daily fluctuations are reduced until at two or three feet
they are not apparent. Seasonal air temperatures also lag as is indi-
cated by the usual hot days of July and the cold of January, both
extremes coming after the June and December solstice. Soil tem-
peratures follow seasonal atmospheric trends with a further lag.

Since the total insolation decreases with distance from the Equa-
tor, temperatures likewise decrease. Temperature zones, therefore,
tend to run east and west, and the greater the latitude the lower
the temperatures to be expected.

There are, however, local and generalized exceptions. Large

CLIMATIC FACTORS : RADIANT ENERGY

123

day night day night day night

Time of Readings

day

NIGHT

DAY

FlG. 64. Maximum day and night soil temperatures taken on a sand dune
at Beaufort, N. C. in August, 1947. Readings were made on successive days
at 7:00 A.M. and 7:00 P.M. for night and day maxima, respectively. Tempera-
tures were greatest at the soil surface and were successively less with increas-
ing depth by day, but, at all depths at night, dropped as low, or lower than
the maximum at eighteen inches. Minimum temperatures fluctuated within
the range of 72-85° F. (difficult to show accurately on so small a scale). At
eighteen inches the minimum was never more than, one degree below the
maximum, but the difference between minimum and maximum increased up-
ward to the surface where one minimum was as low as 72° F.

bodies of water are slower to warm up and slower to cool than
land because of the higher specific heat of water. In addition, they
reflect much of the insolation, and what heat is absorbed is dis-
tributed to much greater depths by water motion and convectional
currents. As a result, temperature extremes are reduced around
bodies of water as compared to those inland. The effect on plant
distribution is particularly evident in the ranges of southeastern
species, which often extend to the northern limits of the Atlantic
coastal plain, where, undoubtedly, they are able to survive because
of the maritime climate. The amelioration of temperatures is ap-
parent about lakes as well as oceans, although to a lesser extent.
The extremes of winter and summer temperature characteristic of
the Dakotas are never experienced in lake-bounded Michigan, al-
though latitudes are essentially the same.

124 THE STUDY OF PLANT COMMUNITIES • Chapter VI

The air near the earth's surface is warmed by absorption of
insolation and reradiated heat from the earth. With increasing alti-
tude the atmosphere becomes less dense and also contains less
moisture and other heat-absorbing substances. Consequently, tem-
peratures decline with altitude. Even the warm air rising from the
earth is cooled by its expansion. Latitudinal temperature zones are,
therefore, further disrupted by mountains where increasing alti-
tude produces the same differences as increasing latitude. This is
particularly noticeable on high mountains where, because of the
combined effects of temperature and moisture, one may see zones
of vegetation altitudinally arranged, which at lower altitudes are
latitudinallv distributed over hundreds of miles.

Just as latitudinal temperature zones are irregular, so are the alti-
tudinal zones not perfect. Cold air drainage has been discussed,
(p. 98.) It results in low night temperature in the valleys when
tablelands and upper slopes are much warmer.129 The areas may be
distinctively marked by the vegetation they support. In moun-
tainous country, orchards are frequently grown successfully at
much higher altitudes on slopes than in valleys.81 Slope and expo-
sure disrupt mountain temperature zones even more. Since the
maximum effectiveness of insolation comes only when it strikes
a surface at right angles, the greater the variation from a ninety-
degree angle, the less radiant energy will strike a unit area. In the
Northern Hemisphere, therefore, a south-facing slope receives
more insolation per unit area than a flat surface, and a north-facing
slope receives less (see Fig. 67). Thus the same temperature con-
ditions found on a tableland may occur at a higher altitude on a
near-by south-facing slope and at a lower altitude on a north slope.
The distribution of vegetation being correlated with temperature
and the consequent moisture differences, a particular community
will be found above its ordinary altitudinal range on south slopes
and below it on north slopes, and the extent of this irregularity in
zonation is affected both by the angle of the slope and its exposure.
In Wyoming, Douglas fir from the montane zone may come down
to 7,500 feet on north-facing slopes while mountain mahogany
from the lower woodland zone may be found extending upward to
better than 8,500 feet on south-facing slopes. In general, a vegeta-
tion zone extends higher on the south side of a mountain than on
the north side.

CLIMATIC FACTORS : RADIANT ENERGY

125

I4O00

12000

10000

u
ui

8000

6000

4000

2000

FlG. 65. A generalized profile of altitudinal zones of vegetation in the
mountains of Utah, which illustrates the effects of northern and southern ex-
posures.— Adapted jrom Woodbury .

276

Cover and Temperature.— Anything that absorbs or reflects in-
solation before it reaches the earth will reduce both soil and atmos-
pheric temperatures. Thus it is cooler in cloudy or foggy areas
than in similar areas without clouds or fog, and any given area
tends to be warmest on clear days. But, because heat radiated from
the earth and clouds is held below a cloud blanket, the lowest tem-
peratures also occur on clear days, and extremely low temperatures
are not to be expected on cloudy days. Temperatures in and above
bare soil, particularly dark soil, are higher than if that soil has some
form of cover. Any type of vegetation must absorb some radiant
energy and, consequently, reduce temperatures between itself and
the soil, the reduction being proportionate to the closeness of the
stand and how many strata compose it. Temperatures in forest
stands in midsummer are usually ten degrees lower by day than in

126 THE STUDY OF PLANT COMMUNITIES * Chapter VI

the open and ten degrees higher at night. Soil temperatures under
forest are lower than in the open during the growing season and
usually higher in winter. However, soil temperatures under de-
ciduous forest are subject to considerable winter variation.

FIG. 66. Effect of slope exposure is apparent in the desert, as elsewhere.
Although species differences are not great, the south-facing slope at right sup-
ports a much sparser, more widely spaced stand of sagebrush than the oppo-
site slope. Washoe County, New. -Photo by W. D. Billings.

Soil temperatures are further modified by dead or living cover
on the surface. Any such cover reduces the range of extremes and
the speed of variation. This amelioration of temperature may be
important in the viability and germination of seeds and the sur-
vival of seedlings. Particularly affected are the physical and physi-
ological processes involving water, its movement and availability
in the soil, and its absorption and transpiration by the plant. Also,
when soil is frozen, the runoff from heavy rains is much increased.
Studies in Arizona123 showed daily minimum soil temperatures to

CLIMATIC FACTORS : RADIANT ENERGY

127

be five degrees higher under forest litter in the fall of the year
than in bare ground and the daily maximum to be seven degrees
lower. The average diurnal range was eighteen degrees in bare soil
and only six degrees under litter. In North Carolina,165 litter re-
duced the depth of frost penetration 40 percent, and, whereas the

TABLE 7. The average day and night temperatures (°F) in three upland
forest communities in central Iowa. Air temperatures in contiguous prairie
are higher than those in shrub by about 10° (day) and 4° (night). From(4).

Community

Time

April

May

July

August

Shrub

Day

58.8

65.1

76.7

73.9

Night

45.3

52.8

64.0

61.1

Oak-hickory

Day

57.5

63.8

80.1

77.7

Night

42.2

51.9

70.4

68.4

Maple-basswood . .

Day

55.0

60.9

74.3

70.0

Night

40.2

51.8

67.5

61.0

bare soil was frozen solidly, the soil under litter remained porous
and loose, permitting deeper percolation during winter rains and
thaws and causing very little runoff. The effects of snow as an
insulator are much the same as are those of litter.

Temperature and Physiological Processes.— There is probably
for every species an average optimum temperature at which it
grows most successfully, other factors being equal. Likewise there
must be a maximum and a minimum temperature that it can with-
stand. These limits may result from the temperature tolerances of
the protoplasm peculiar to the species, but they may likewise
result from responses of one or more physiological processes,
which vary from species to species.

The temperatures affecting germination might alone limit the
range of a species. Among our cultivated crops, the minimum-
maximum range of temperature for germination is 35°-82° F. for
flax and 49°-115° F. for corn. The optimum for each, respectively,
is 70° and 93°. That the center of production for flax is consid-

128 THE STUDY OF PLANT COMMUNITIES • Chapter VI

erably north of the center for corn is therefore not at all surpris-
ing.

Absorption of water is at a minimum when soil is frozen but
increases, as do diffusion and capillary movement in the soil, with
rising temperature. The optimum is surprisingly high as soil tem-
peratures go, and the maximum approaches the boiling point in
some instances. Absorption is reduced, more at low temperatures
for plants that grow normally in warm soil than for plants that
grow, at least part of the year, in cold soil. For example, cotton
absorbs only 20 percent as much water at 50° as at 77° F. while
collards absorb 75 percent as much at 50° as at 77° E148

Photosynthesis operates under a wide range of temperatures
under natural conditions. Marine polar algae may live their entire
lives at temperatures below 32° F. because the freezing point is
depressed by the salts in the water. There is an often-quoted old
report that spruce carries on photosynthesis at -22° E, but a re-
cent study" using modern methods indicates that, although coni-
fers do not lose their ability to carry on photosynthesis during
midwinter, the species studied function only above 2 1 ° F. The
process also goes on in desert plants at temperatures of 120° E or
more. The effective temperature range, however, is usually be-
tween 70° and 100° E With increase in temperature the rate in-
creases steadily to the optimum and then drops abruptly to the
maximum, which is not much in excess of the optimum. The rate
of respiration also increases with temperature until at high tem-
peratures the process becomes destructive of life. Vant Hoff's
Law, which states that the speed of a chemical reaction doubles
or more than doubles with each 18° F. rise in temperature, is ap-
plicable within limits to reactions in organisms. In photosynthesis
it holds reasonably well between about 41° F. and 77° F. Beyond
these limits there is much variation.

Growth, being a product of chemical and physiological proc-
esses, follows the same pattern and is favored by relatively high
temperatures. At temperatures near or above the maximum, the
water balance is apt to be thrown off by excessive transpiration.
Reproduction follows the same rule regarding temperature, but
it is of interest that flowering and fruiting have higher optima
than vegetative processes in the same plant.

CLIMATIC FACTORS : RADIANT ENERGY 129

LIGHT

That portion of the sun's radiant energy which forms the vis-
ible spectrum and which we commonly term "light" strikes the
earth in quantities far in excess of the apparent needs of plants.
Although green plants, with very few exceptions, are the only
organisms that can directly convert this energy to their own use,
they actually change to potential energy only about one percent
of the light energy they receive. It has been estimated that, of the
total solar energy falling upon a given field of corn during a grow-
ing season, only 0.13 percent can be "stored" as potential energy.
However, this also suggests that, to function normally, plants
require much more light energy than they actually use. Not all
wave lengths are equally usable. Green light is reflected or trans-
mitted, while the longer wave lengths, in the red end of the spec-
trum, are much more effective in photosynthesis than are the
shorter lengths of yellow and blue. Not all species are equally
efficient under equal illumination. Some require certain intensities
and some need certain lengths of day or season to function nor-
mally. To add to the difficulties of interpreting plant-light relation-
ships, it is not always possible to distinguish between light effects
and those of total insolation, which include heat and its influence
on physiological processes.

Light Measurements.— Ecological studies of light should not be
casually undertaken in spite of the apparent simplicity of making
measurements with modern instruments. As suggested above, plant
responses and light values rarely bear a simple and direct relation-
ship to each other. Whether or not these relationships can be in-
terpreted may depend upon proper planning before making meas-
urements. In addition, there are problems related to obtaining
measurements for ecological purposes that must be considered.

Chemical, illuminating, electrical, and heating effects of light
are measurable, and for each a different type of instrument is
used.233 Field ecologists have largely abandoned the first two ap-
proaches in favor of electrical measurements because of the recent
perfection of compact, sturdy photoelectric apparatus with which
accurate and rapid determinations can be made. These instruments
are sensitive to approximately the same portion of the spectrum as
is the human eye. Since they are selective instruments, there may

130 THE STUDY OF PLANT COMMUNITIES ' Chapter VI

TABLE 8. Light measurements, in foot candles, made with a Weston
photometer in a mixed pine-hardwood stand between 12:00 and 1:00 P.M.
when full sunlight was 9,500 foot candles. Readings taken along three lines,
at three-foot intervals, at a height of three feet. After completion of a line,
the measurements were repeated at the same points. Note the great variation
in readings at the same points at different times (sun flecks) and that some
points are apparently much less shaded than others.

Line I

Line 2

Line j

300

500

400

100

300

500

300

200

400

200

300

400

300

100

200

300

400

200

600

300

200

300

200

300

100

200

400

3000

200

300

100

200

3600

2400

200

400

2400

200

4400

3600

300

200

400

200

100

200

300

200

2000

400

200

2400

400

100

200

500

200

400

300

100

200

500

200

600

400

100

500

200 *

1000

800

100

200

300

200

300

200

300

200

400

1600

200

4200

200

300

200

100

200

1200

5000

100

200

2100

1600

2400

200

100

200

600

300

4400

100

300

100

400

500

100

200

300

100

200

300

200

400

200

800

2000

200

100

Aver. 532

432

832

1140

224

200

Average for the stand = 560 ft. candles. 5.9% of full sunlight.

be some question of the advisability of generalizing as to plant
responses in relation to the measurements they obtain. In most
field studies this does not become a serious limitation because the
usual objective is to compare relative intensities of light in two or

CLIMATIC FACTORS : RADIANT ENERGY 131

more situations or habitats. For this purpose, the photoelectric
method is quite usable.

The method has, however, other limitations, and its use requires
certain precautions. Preferably two or more instruments should
be available and the readings should be made simultaneously. Even
so, readings should be made only on a clear day and, when pe-
riodic observations are made, at the same time of day. Results
should be expressed as percentages of full sunlight at the time
when each observation is made. At sea level this would be approxi-
mately ten thousand foot candles on a clear day at noon, but values
as high as twelve thousand foot candles have been obtained in the
clear air of high mountains. If for any reason the readings in the
open are low on a given day, no further observations should be
made.

Because of its concave sensitive surface, the instrument can be
operated in only one plane at a time. If readings are made simul-
taneously at noon with the instrument in a horizontal plane, many
complicating factors are automatically eliminated. The instru-
ments are extremely sensitive to slight variations in light, and this
necessitates numerous readings to arrive at average conditions.
The slightest air movement shifts the position of leaves and per-
mits bright sun flecks to come through a forest canopy. These
flecks come and go, first at one point and then at another, and
cannot be ignored in evaluating light in a stand. Their inclusion is
best accomplished by making observations at a rather large num-
ber of uniformly or randomly distributed predetermined points
and averaging the results. In all instances, the instrument should
be in the same position relative to the observer and the ground.

A sensitive surface of spherical form is usually more desirable
than a flat one. Where reflected light is appreciable, a sphere will
record from all directions. If a continuous record is to be obtained,
the sphere records accurately because one-half its surface always
faces the sun regardless of its position. Several radiometers, which
measure heat effects and are nonselective of wave lengths, are
spherical in form and are advantageous in other respects. If a
photoelectric cell is given more than a short exposure to strong
light, the current it generates falls off because of solarization, but
the radiometer can be exposed indefinitely without such effects.

132 THE STUDY OF PLANT COMMUNITIES • Chapter VI

It is, therefore, adaptable to continuous operation with a record-
ing device.

Such equipment is not always available to the field ecologist,
but, even so, some form of measurement is far more dependable
than an estimate. Good approximations of light intensity may be
obtained with photographic light meters even though they are not
calibrated in foot candles. Useful values are obtainable by expos-
ing black and white bulb atmometers in pairs. When one pair is
exposed in the open and differences from pairs in near-by habitats
are expressed as percentages of the value in full sunlight, the re-
sults may be quite as satisfactory as with more elaborate equip-
ment. Since the atmometers would be operating continuously,
they might even be more meaningful in terms of the vegetation.

Light Variations.— The biologically important variations of light
are those in intensity and quality. These occur periodically, re-
curring seasonally and daily to a degree that is determined by
latitude140 as discussed under the general heading of insolation.
Of course, altitude modifies the regional variations, and topog-
raphy results in more localized variation through the effects of
angle of slope and direction of exposure. Since the principles were
previously discussed (p. 124), it should be sufficient here to pre-
sent an illustration of how slope and exposure affect light in the
southern Appalachian Mountains.41

Variation in quality of light is not so obvious as variation of
intensity. Quality, however, is variable, largely because of the
same factors that modify intensity, for the amount of absorption
and diffusion by the atmosphere determines what wave lengths
reach the earth. Clouds, fog, smoke, dust, or atmospheric moisture
alone increase diffusion and absorption, and, as a consequence, dry
regions receive more light than humid ones, and open country
receives more light than smoky cities. The greater the diffusion,
the higher the percentage of red light and the lower the percent-
age of blue reaching the earth.

A local variation of far greater general ecological importance is
that produced by vegetation of one stratum upon that of a lesser
stratum beneath it. Because plants growing in the shade of others
receive only the light that is not absorbed or reflected, they must
be adapted to functioning with reduced light intensity (often re-

CLIMATIC FACTORS : RADIANT ENERGY

133

6AM 8AM 10AM NOON 2 PM 4 PM 6 PM

TIME OF DAY

FlG. 67. Intensity of radiation received at different times of day on (A)
south, (B) north, and (C) east slopes in the southern Appalachians, on June
21 and on December 21. For S. exposure, in summer, the 20 percent slope
receives greatest radiation because it forms an angle of almost 90° with the
sun's rays at noon. In winter, when the sun is low, the 100 percent slope re-
ceives more radiation than the 20 or the 40 percent slope. For N. exposures,
in summer, 20 percent slopes receive almost as much radiation as 20 percent
south slopes. In December, 100 percent N. slopes are in complete topographic
shade but 100 percent S. slopes receive 48 percent of maximum radiation at
noon. Curves for west slopes would be mirror images of those for east slopes.
—From Byram and Jemison.**

duced to 15 percent or less) of somewhat different quality (re-
duced red and blue light) than those in full sunlight receive. Con-
sequently, there are species representing a wide range of tolerance
to shade, for no forest is so dense that nothing can grow beneath
it, even when there is a reduction to 1 percent or less of full sun-
light, as under some tropical forests. The reduction of light in-

134 THE STUDY OF PLANT COMMUNITIES * Chapter VI

tensity under a forest .canopy is probably of more ecological im-
portance than the change in quality.

Shade Tolerance.— The ability or inability of certain plants to
grow normally when shaded, as on the forest floor, has several
practical considerations. When a forest stand is thinned or clear-
cut, the new stand that appears will, in general, be determined by
the kinds of seedlings and saplings already present at the time of
cutting. These species may or may not be desirable, and the ques-
tion of how to encourage or inhibit them, depending upon circum-
stances, has led to much study and theorizing on the causes of
shade tolerance.

Since light is obviously reduced under a forest stand, it was
once assumed rather generally that light is the controlling factor.
Studies of "trenched plots" under forest stands gave results inter-
preted by many workers as indicating a greater significance for
water since, within these plots, shade-intolerant species for a time
grew well when root competition for water and nutrients was
eliminated by cutting off the roots of the dominant trees.146' 253
Extensive investigations of conifer reproduction in the Lake States
indicate that, for each light intensity, growth could be increased
by reducing root competition and that at each level of root com-
petition growth could be increased by increasing light.234 Obser-
vations of the reproduction of certain southern pines190 indicate
that these shade-intolerant species may successfully meet extreme
root competition if light is sufficient. It would seem that the suc-
cessful growth of a seedling under a forest canopy may depend
upon its ability to manufacture enough food with the light avail-
able to grow enough roots to meet the competition of the trees
established there. Undoubtedly, shade tolerance cannot be ex-
plained on the basis of a single factor.

Physiological Responses.— When the supply of food in an or-
ganism falls and remains below what is required for respiration
and assimilation, the organism cannot continue to function nor-
mally and must eventuallv die. Since a green plant produces its
carbohydrates through photosynthesis, the process must proceed
at a rate sufficient at least to satisfy the immediate needs of the
plant if growth is to be normal. Light, which provides the energy
for photosynthesis, is sufficient during the growing season to sup-

CLIMATIC FACTORS : RADIANT ENERGY

135

ply plant needs anywhere on the earth. In fact, light intensities
may be too high for some plants to grow in full sunlight, their
seedlings being especially subject to injury. Such plants might well
be restricted to habitats with partial shade; if their photosynthetic

FlG. 68. Trenched plot in a loblolly pine stand (40T50 yr.) four years after
initiation (see Fig. 12). Contrast vegetation on trenched plot with floor of
surrounding forest and control plot in foreground.— Photo by C. F. Korstian

146

efficiency is insufficient to maintain them in forest shade, they
might thrive in regions where light intensity is reduced by cloudi-
ness or fog. Probably the range of a species is rarely determined
by light intensity alone, however, for it must be remembered that
light effects are apparent in several processes and activities, which
can rarely be considered independently. The production of chlor-
ophyll, the opening and closing of stomata, and the formation of
auxins are examples of light-conditioned phenomena with widely
differing effects, but these activities must be considered in rela-
tion to each other when interpreting plant responses.

The production of chlorophyll, although (with a very few ex-
ceptions) accomplished only in the presence of light, is perhaps
more apt to become limiting or significant in high than in low
light intensities. Available evidence indicates a greater production

136 THE STUDY OF PLANT COMMUNITIES * Chapter VI

of chlorophyll with decreasing light intensity and an ability of
most plants to produce chlorophyll at light intensities considerably
below those necessary for effective photosynthesis.

The opening and closing of stomata can usually be correlated
with light, but there are enough exceptions to give warning against
generalizations or interpretations based on the principle of alter-
nate opening and closing with light and darkness. In some plants,
stomata may open at night; in others, light seems not to be a con-
trolling factor. Where stomatal movement seems directly respon-
sive to light, other factors may at any time become more impor-
tant and modify or counteract the effects of light, as when stomata
close during the day if the water supply is insufficient. However,
stomatal movement is usually correlated with light changes and,
when other conditions are favorable, is apparently caused by tur-
gidity changes in the guard cells resulting from metabolic activity,
which varies with light. The opening and closing in turn may
modify effects of light by varying gas exchange related to photo-
synthesis and rate of loss of water by transpiration.

The production of certain auxins or growth-controlling sub-
stances in plants is inhibited by light. As a result, through them,
size, shape, movements, and orientation of parts may be influenced
by light. A plant grown in complete darkness, since it produces a
maximum of auxins, elongates excessively, with poorly differenti-
ated tissues throughout and with almost no supporting structure.
These characteristics in an intermediate condition are often rec-
ognizable in plants grown in heavy shade, as under a forest canopy
or in close stands where plants shade each other. Such plants tend
to be tall and spindly with widely spaced nodes and relatively few
leaves. The better the light, the stouter and more compact the in-
dividual will be.

Should illumination be one-sided, the increased production of
auxins on the shaded side usually stimulates sufficient extra elonga-
tion on that side to turn the growing portion of the stem toward
the light. Some species— sunflower, for instance— are so sensitive
to such differences of light that the floral portions shift from east
to west with the sun daily as differential elongation in the stem
progresses from one shaded side to the other.

The orientation of vegetative parts is such that every leaf re-

CLIMATIC FACTORS : RADIANT ENERGY 137

ceives a portion of the light available. Genetic differences deter-
mine whether the leaves are exposed in the form of a rosette or
in a mosaic pattern, or whether they are supported by a spirelike
central axis or several spreading branches, each of about equal
size. The variations within such a general plan probably result
from effects of auxins on growth of petioles and secondary
branches.

Leaves normally become arranged with their broadest surface
exposed outward and upward on the side of the plant where they
grow. This results in a maximum exposure to the available light
at that point. However, plants growing under conditions of ex-
cessive light, especially where there is reflection from light-colored
soil, not uncommonly have their leaves in a profile position, which,
of course, reduces the light to which they are exposed. Turkey
oak (Quercus catesbaei), which grows on sand dunes in the south-
eastern United States, regularly develops a twist in the petiole
that turns every blade vertically. The leaves of wild lettuce (Lac-
tuca scariola) are vertical when grown in full sunlight but do not
change from a horizontal position in the shade. Several so-called
compass plants have leaves that are not only vertical but that also
face east and west, exposing only their edges to the sun's rays at
midday.

Plants growing in close stands characteristically lose leaves and
usually branches from below when the light penetrates insuffi-
ciently to maintain necessary photosynthesis. Most monocots with
grasslike leaves and underground stems are unaffected because
their upright linear leaves permit light to penetrate to their bases.
In forest stands, this self-pruning may be economically impor-
tant. Conifers that self-prune grow tall and straight with few
knots and smooth grain. In contrast, those with dead branches
down to their bases are difficult to handle and produce much less
valuable wood when finally cut.

Leaves grown in full sunlight tend to be smaller, thicker, and
tougher than leaves grown in the shade. This is particularly no-
ticeable in plants of the same species and may also be observed on
the same plant. A forest-grown tree may have sun leaves at the top
and shade leaves near the base, or in the interior of its crown.

Certain structural differences are associated with the two types

138 THE STUDY OF PLANT COMMUNITIES ' Chapter VI

of leaves. Intense light results in elongated palisade cells and often
the production of two or more layers of them. Conversely, weak
illumination favors the production of sponge cells. A leaf that,
with average illumination, has a single layer of palisade and several

FlG. 69. Seedling of turkey oak (Quercus catesbaei), a sandhill species,
whose leaves have already assumed the vertical position they maintain
throughout life.

layers of sponge cells might have had, in intense light, two or three
layers of palisade and a proportionate reduction in sponge tissue.
In reduced light the sponge tissue is increased at the expense of
the palisade. In extreme cases there may be no palisade or no
sponge tissue. The thickness of cutin and the amount of support-
ing tissue in the veins are likewise greater or less depending upon
light intensity. These characters affect the relative toughness of
the leaf.

What forces cause a developing cell to elongate at right angles
to the leaf surface to form palisade or parallel to the surface to
form sponge tissues, cannot be stated with any certainty. The
causes may not be entirely controlled by light, for unfavorable
moisture conditions favor palisade production as does poor aera-
tion. Sucker sprouts from stumps often produce leaves of the shade

CLIMATIC FACTORS : RADIANT ENERGY

139

type in full sunlight, probably because of the favorable water bal-
ance maintained by the extensive root system of the tree. Certain
advantages of shade leaf development are more obvious than the
causes.

In strong light, cells elongate parallel to the light source. The

FlG. 70. The anatomical characteristics associated with so-called sun and
shade leaves of two chaparral species. (A) Arctostaphylos tomentosahom nor-
mal xeric habitat, (B) from mesic habitat. (C) Adenostoma jasciculatum from
normal xeric habitat, (D) from stump sprout. Note differences in thickness of
leaf and cuticle, and proportion of palisade to sponge tissue.— From Cooper™

more intense the light, the deeper its penetration into the leaf and
the more layers of palisade there will be. Desert and alpine plants
may have the mesophyll entirely made up of palisade cells. Leaves
subject to reflected light from below commonly have palisade on
the lower surface as well as the upper, and leaves growing ver-
tically regularly have palisade on both sides.

When illumination is intense, chloroplasts arrange themselves
along the side walls, and thus in palisade cells they receive a mini-
mum of direct insolation. On the other hand, with weak light the
chloroplasts tend to appear along the walls at right angles to the
light source, and the form of sponge cells permits exposure of
more chloroplasts to the greatest effectiveness of available light.

140 THE STUDY OF PLANT COMMUNITIES ' Chapter VI

There are added advantages (in the thinness and greater area of the
shade leaf since both maximum exposure under conditions of re-
duced light and penetration of light to a high proportion of in-
ternal cells are thus assured.

Since reduced light favors elongation, vegetative growth, and
delicacy of structure, it can readily be understood why several

A b c

Fig. 71. Structure of leaves of broad sclerophyll forest trees (A) Castan-
opsis chrysophylla, (B) Quercus agrifolia, (C) Quercus durata. Note com-
pact structure, multiple layers of palisade, and tendency for all mesophyll to
be palisade-like. Note also struts of mechanical tissue from epidermis to epi-
dermis.— From Cooper™

garden crops used either for leaves or roots are best grown in
spring and fall or in regions with many cloudy days. A number of
leaf crops are grown under artificial shade. The point is well illus-
trated by the production under artificial shading of the large thin
leaves of tobacco needed for cigar wrappers.

Since intense light inhibits vegetative growth and favors, or is
actually necessary for, flowering and fruiting, it is not surprising
that centers of grain and fruit production characteristically have
much clear, cloudless weather during the growing season. Here,
too, is a partial explanation of the reduced size of alpine and arctic
plants, which produce large and numerous flowers. Likewise it
helps explain why trees in the open often fruit more prolifically
than those in a closed stand, where overtopped individuals rarely
produce a seed crop.

Photoperiod.— A number of seasonal biological phenomena long
have been accepted as such, without much concern as to causes.
Violets, miliums, bellworts and many other wildflowers blossom

CLIMATIC FACTORS : RADIANT ENERGY 141

in the spring, but asters, goldenrods, and chrysanthemums are ex-
pected to flower in late summer or fall. When a fruit tree occa-
sionally blossoms in the fall, the occurrence is considered unusual.
The controlling factor in such periodic phenomena was not recog-
nized until Garner and Allard104 published results of their studies
of photoperiodism, or responses of organisms to the relative length
of day and night. Their investigations developed from difficulties

FlG. 72. The effect of long day (15 hours), left, and short day (9 hours),
right, on flowering of henbane {Hyoscyamus niger), a long-day plant. All
plants received 9 hours of natural radiation. The supplemental light of the
15-hour lot was obtained from 100-watt incandescent lamps, which gave an
intensity of only about 30 foot candles.— Photo by courtesy of H. A. Borth-
ivick, Bureau of Plant Industry, U. S. Dept. Agr.

experienced in growing new varieties of tobacco and soy beans in
the vicinity of Washington, D. C. The tobacco grew vigorously
and did not flower under field conditions, but in the greenhouse,
during the winter months, it flowered and fruited abundantly.
The soy beans flowered and set fruit at about the same date in late
summer regardless of how long they had been in the vegetative
condition, as determined by plantings spaced at wide intervals
during the spring and early summer. When the length of daylight
period was shortened for these plants by enclosing them in a dark
chamber for a few hours each day, the tobacco flowered very
soon and the formation of seeds in the soy beans was hastened
materially.

Some Applications '.—It can readily be seen why garden plants
grown for vegetative parts, if they are long-day species, develop
best in spring and late fall and, if grown in summer, bolt to form

142 THE STUDY OF PLANT COMMUNITIES • Chapter VI

Fig. 73. An Abelia hedge in late fall that (left) ceased growth and hard-
ened normally everywhere except section under boulevard light. Here, be-
cause of the extended photoperiod, the plants continued to grow and put out
new shoots, which were killed by the first heavy frost (right).— From
Kramer.1*1

flowering structures. The differences in photoperiodic response
between varieties may be the sole reason for success or failure of
a crop at a particular latitude and is an excellent reason for know-
ing one's seed stock and its potentialities. Flowering shrubs and
herbs, too, if grown beyond their normal latitudinal range, may be
pampered and kept alive but often fail to flower because the length
of day is unsuitable, or may invariably flower too earlv in the
spring or too late in the fall.

The cessation of growth and subsequent "hardening" of ever-
green woody plants are initiated in response to length of day. If
plants are put out within range of street lamps, some winter-killing
mav be anticipated. Street trees of several species retain their leaves
on the side illuminated by street lamps long after dormancy and
complete leaf fall on the opposite side, which does not have sup-
plemental light.171 On the Duke University Campus, lamp posts
are regularly spaced in a long Abelia hedge, and every winter frost
injury results within a certain distance of each lamp because the
plants here do not go into dormancy.147

CLIMATIC FACTORS : RADIANT ENERGY 143

Commercial greenhouses are making use of supplementary light-
ing and controlled period of illumination to bring crops into
flower on special days or to produce maximum vegetative growth.
Growing a crop for its vegetative parts in one latitude for which
seeds must be produced in another latitude is now common prac-
tice.

Ecological Significance?— It is thus apparent why many plants
in the tropics, where the light period is almost constantly twelve
hours, flower throughout the year and, likewise, why so few
plants in the United States, even in the South, have this character-
istic. It is apparent, too, that arctic species must be long-day plants
and why they rarely flower when brought farther south. Also,
short-day species could not survive in the tropics since they would
not reproduce. Species requiring high temperatures and long days
to mature are definitely limited in their northern range. The for-
mation of abscission layers in leaves of trees and their decline in
physiological activity are initiated in response to shortening days,
not to reduced temperature. Therefore, at or beyond the northern
limits of their range, trees may be killed by frost because they are
not yet sufficiently dormant to withstand low temperatures when
they occur.

It should not be assumed that plant distribution is primarily de-
termined by length of day. Many species are little affected by it.
Also temperature fluctuations have been shown to modify photo-
periodic requirements and responses in several species. Photo-
period is just another factor, which may operate with temperature,
moisture, and light to determine the range and distribution of a
species.

GENERAL REFERENCES

H. A. ALLARD. Length of Day in Relation to the Natural and Artificial

Distribution of Plants.
R BURKHOLDER. The Role of Light in the Life of Plants.
R. F. DAUBENMIRE. Plants and Environment. New York : John Wiley and

Sons Co., 1948. 424 pp.
W J. Humphreys. Ways of the Weather.

H. L. SHIRLEY. Light as an Ecological Factor and Its Measurement.
U. S. DEPT. Agr. Climate and Man.
H. B. Ward and W E. Powers. Weather and Climate.

CHAPTER VII

PHYSIOGRAPHIC FACTORS

SOIL

Land masses of the earth are covered by an unconsolidated sur-
face mantle of mineral particles derived from parent rock by proc-
esses collectively called weathering. The depth of the mantle is
variable depending upon disturbances and time, while its physical
and chemical properties depend upon the nature of the parent
rock and the weathering agencies that may have affected it. This
inorganic material may be termed soil but is usually not so con-
sidered until organic materials have accumulated from organisms
that have lived in or upon it.

Soil Formation.— Weathering may result in purely physical
change, as when rock masses are broken into smaller and smaller
sizes, or may be of a chemical nature, producing changes in com-
position of the material. The two processes function together nor-
mally. Disintegration is largely accomplished by physical agents,
such as water, wind, ice, and gravity, and by expansion and con-
traction resulting from temperature changes. The first four agents
are functional through the erosive action of the load of cutting
material they transport and are, therefore, effective in proportion
to speed of movement or to force and pressure. The effects of
temperature are the most widespread although not always con-
spicuous. Differential expansion and contraction of rock materials
result in cracking, which is especially marked when temperature
changes are abrupt. The widest temperature fluctuations occur in
arid regions and at high altitudes where their effectiveness is indi-
cated by consistently coarse and angular soil particles. To a lesser
extent the process goes on everywhere. Prying action of plant
roots and excavating or burrowing by animals may contribute to
disintegration, but these activities are certainly of greater impor-
tance in their facilitating of chemical processes. Openings in the
soil increase aeration and the percolation of water. Shifting the

144

PHYSIOGRAPHIC FACTORS

145

Fig. 74. Wind-swept alpine habitat in Utah with typical coarse, angular
soil particles and little organic material. Krummholz at left is of Picea en-
gelmanni and Pinus flexilis (see also Fig. 47).— U. S. Forest Service.

soil about exposes new particles to chemical action and likewise
helps to incorporate organic matter.

The chemical or decomposing processes all tend to result in in-
creased solubility of soil materials, which, in solution, may then be
available for the use of plants but are also subject to leaching, or
washing out, of the surface layers by rain water. Both oxidation
and hydration, the addition of oxygen or water to a compound,
are common and result in softening of rock. Carbonation, or the
taking up of carbon dioxide, produces carbonic acid merely by
union with water, and the acid is an effective solvent of many
rocks. Water itself is a weak solvent, and, with the addition of
carbonic acid, which is always present, its action is much increased.
Decaying vegetation, when present, also contributes acids that
facilitate solution. In solution, salts ionize and the relative effective
concentrations of the basic and the acid radicals thus formed de-

146 THE STUDY OF PLANT COMMUNITIES ' Chapter VII

termine whether the soil solution will be alkaline or acid in reac-
tion.

These and other chemical processes operating more or less con-
tinuously, together with physical processes, constitute weathering,
which produces soil material that retains few characteristics of
parent rock. However, soil is not a product of these processes
alone, for biological activity also contributes to its formation. Or-
ganic material is an essential part of soil, and its decomposition and
incorporation are accomplished largely by microorganisms, whose

Fig. 75. A soil well that illustrates a soil profile (White Store sandy loam)
in which the A0 horizon is very thin, the sandy gray-white A horizon is
sharply distinguished from the plastic red clay of the B horizon, and the
rocky C horizon shows in the bottom— Photo by C. F. Korstian.

PHYSIOGRAPHIC FACTORS 147

numbers and activities increase as more complex organisms, par-
ticularly higher plants, gradually occupy the surface.

Soil Profile.— Processes resulting in the formation of soil mate-
rial also contribute to soil development. As weathering proceeds,
fine materials in suspension and solution are carried downward by
percolating water to a lower level, where they gradually accumu-
late. As a soil develops, therefore, a rough stratification becomes
apparent in which the horizons characteristically have different
physical and chemical properties. These horizons, collectively
called the soil profile, are designated and recognized as follows:

A Horizon. The upper layer of soil material from which
substances have been removed by percolating water.

B Horizon. The layer below the A Horizon in which these
materials have been deposited. Layer of accumulation.

C Horizon. The underlying parent material, relatively un-
weathered and not affected as above.

Litter accumulated on the surface of the mineral soil may be
termed the Ao Horizon. It is often convenient to subdivide the

ROOT DISTRIBUTION ON OUTSIDE WALL OF SOUTH TRENCH
TRENCHEO PLOT NO 2 JUNE 10,1932.

A> • : — 1 5 s"

9 5'

•

• " •

•

• •

•

• * •

• •

V '..-■ — : — '

l' ' **
• • .

•

•
•

•

• — "-» " ,

•

• 6 .

•

• •

•

i 5

B. • •.••. - 215*

360*
LEGEND
• = root.0'-or ..root 0.2*- 0.3" ■ SCALE ,
.»roo10.f-0.2' •=root0.3'-0.4* "cot

FlG. 76. An illustration of root distribution in soil horizons and of a
method for mapping roots in the wall of a soil well.— From Korsticm and
Coiled

major horizons as Ai and A2, Bi and B2, etc. Ai is a particularly
useful subdivision, for it is applied to the portion of the A horizon,
distinguishable by its darker color, in which organic material has
become incorporated.

Soil profiles may be observed in any fresh road cut. When
studied in connection with vegetation, a rectangular pit is usually

148 THE STUDY OF PLANT COMMUNITIES • Chapter VII

dug some four to six feet long, and wide enough to stand in com-
fortably. One face is kept vertical and cut cleanly to observe the
horizons— and possibly the root distribution. Depth of the pit is,
of course, determined by local conditions and position of the par-
ent material.

Soil-Plant Relationships.— Soil must provide plants with an-
chorage, a supply of water, mineral nutrients, and aeration of their
roots. Not all plants require these essentials to the same degree, but
unless all are present to some extent the average plant cannot be-
come established. On this basis, soil has four major components :
(1) mineral material derived from parent rock, (2) organic sub-
stances added by plants and animals, (3) water, and (4) soil air.
These components vary in amount and proportion from place to
place, and the variation may be a significant factor in determining
the distribution of species and vegetation types.

Local Soil Variations.— Size of soil particles (soil texture) and
shape of particles, which determines how they fit together (soil
structure), may vary markedly within short distances. Texture
and structure primarily affect the plant through their influences
on air and water in the soil. Organic materials, in addition to
modifying soil structure, are the source of plant nutrients that
may be quite unavailable from mineral sources.

These variables are a product of the manner in which the soil
originated and the time involved in its development. Great areas
of the earth are covered with soils that overlie the parent rock
from which they were formed. These are sedentary soils, whose
materials are termed residual, if of mineral origin, or cumulose,
when deposited as organic matter. If soil material has been brought
to its present location by some agency such as wind, water, grav-
ity, or ice, it is said to be transported and will accordingly have
distinguishing characteristics.

Soils Formed in Place — Residual materials are most weathered
at the surface and become progressively more like the parent rock
with increasing depth. Where parent rocks differ in hardness or
solubility, the resulting soils will differ. Fine-textured clayey soils
may represent the leached residue of easily soluble rock, such as
limestone, or may be the individual particles that made up a fine-
grained hard rock. When the parent rock contains a high propor-

PHYSIOGRAPHIC FACTORS

149

tion of hard, insoluble material like quartz, its soils will be sandy
or even coarser.

Cumulose materials may be mixed with mineral soils in any
proportion or may have accumulated as almost pure organic
masses. The latter are illustrated by peat bogs, which are common-

FlG. 77. A wide flood plain in an old river valley whose alluvial soils con-
stitute the best farming land in the region. Hiawassee River, Tenn— U. S.
Forest Service.

ly made up of plant remains that only partially decayed and were
added to year after year until the lake or pond in which they grew
was completely filled. Found most abundantly in lakes produced
by glacial topography, the peat accumulations are likewise great-
est where temperatures are low enough to limit the activities of
organisms that produce decay.

Transported Soils— On the great part of the earth's surface
covered with residual soil, the effects of transporting agents are
commonly noticeable only locally. But, to the ecologist, these lo-
calities are of interest because the soil conditions are usually differ-
ent enough to cause vegetational differences too.

Except for loess, discussed elsewhere (p. 112), soils of aeolian
origin are usually sandy deposits, which wind picked up from
wide exposed beaches of lakes or oceans. Normally occurring as
dunes, they usually form unfavorable habitats because of the low
water-holding ability of sand, its relative sterility, and because the

150 THE STUDY OF PLANT COMMUNITIES * Chapter VII

dunes are subject to blowouts should the surface cover of vegeta-
tion be incomplete (see Figs. 55, 56). In contrast, stabilized dunes
of arid or semiarid regions form relatively favorable habitats be-
cause almost all the water that falls upon them is available for
plant use.

Alluvial soils have been deposited by streams, which, as trans-
porting agents, are effective in proportion to their velocity and
the size of particles involved. Since currents are rarely constant,
the size of transported particles varies, and deposits are always
noticeably stratified. Alluvial soils are characteristic of lowlands
that formed as deltas in or at the mouths of streams or as flood
plains along streams that periodically overflow their banks. The
greater the distance from the main channel of the stream, the finer
the texture of the soil materials deposited. Alluvial deposits usually
make desirable agricultural land if properly drained, and, because
of favorable moisture conditions, they usually support the richest
natural flora of a region.

Colluvial materials are transported by gravity. Except in regions
of rugged topography or in mountainous areas, they are rarely ex-

FlG. 78. Colluvial cones, still in formation in Colorado. Only in such
rugged mountain topography is gravity of direct significance in soil trans-
port.— U. S. Forest Service.

PHYSIOGRAPHIC FACTORS

151

tensive. Generally, they occur as talus slopes at the bases of cliffs
from which the material has fallen. They are usually potentially
good soils because they are mixtures of coarse and fine materials,
often originating from several kinds of rocks, and organic matter
is likewise mixed with the mineral components. The favorableness
of the habitat is primarily determined by the moisture supply,
which is strongly variable, depending upon exposure.

Glacial ice plucks and gouges quantities of soil material from
whatever surface it traverses. Carried in the ice, these materials
are ground, pulverized, and mixed until they are deposited as
moraines at the limit of advance or dropped as the ice recedes.

FlG. 79. Shrinkage upon drying as illustrated by some Piedmont soils.
Samples obtained in place (see Fig. 83), then initially saturated with water
and oven-dried. B horizon clays— (1) Orange, (2) White Store, (3) Tirzah;
A horizon sandy loam— (4) White Store. Such shrinking and swelling in the
B horizon affects soil aeration and water movement.— From Coile.04

152 THE STUDY OF PLANT COMMUNITIES * Chapter VII

The glacial debris is heterogeneous in composition and texture,
and the depth of its deposit is highly variable. Drainage is imper-
fect, but melt water from the receding ice is plentiful. Its early,
rapid, and haphazard flow results in the transporting and assorting
of a large amount of soil material, which, as drainage lines become
established, is deposited to form topographic and soil features as-
sociated with glacio-fluvial activity. The water-assorted soils de-
posited in the valleys of glacial streams or carried from terminal
moraines to form outwash plains are characteristic.

Although glacial deposits may include weathered rock and some
organic material, these are usually not abundant in the beginning.
Weathering and the establishment of vegetation at first proceed
slowly on glacial soil, but as they progress, a generally good, pro-
ductive soil is formed. The soils of the northeastern United States
and most of Canada are almost entirely of glacial origin.

Soil Texture.— One of the most useful bases for classifying soils
is that of size of particles. The local variations discussed above are
all reflected in soil texture, which in turn has much to do with soil
moisture, aeration, and productivity.

The standard classification in the United States is that of the
United States Department of Agriculture, which recognizes the
following sizes of soil particles by name:

Name Diameter, mm.

Fine gravel 2.00 -1.00

Coarse sand 1.00 —0.50

Medium sand 0.50 -0.25

Fine sand 0.25 -0.10

Very fine sand 0.10 -0.05

Silt 0.05 -0.002

Clay < 0.002

The percentage weight of these size classes in a soil sample is
determined by mechanical analysis. The larger classes may be
separated satisfactorily by means of sieves, but the fractions of
small size are determined by the pipette method182 or, better still,
the use of a hydrometer.26, 27> 28 Both methods are based upon the
differential rate of settling of particles in water.

After mechanical analysis, accurate textural description is pos-

PHYSIOGRAPHIC FACTORS 153

sible by using the names for the fractions singly or in combina-
tion. The soil classes are named primarily for the predominating
size fraction,87 but when many sizes are present, the term, loam,
is introduced. Thus a soil may be termed gravel or clay if either
of these sizes is present almost exclusively, but if gravel or clay
merely predominates and is mixed with several other size classes,
the soil is called gravelly loam or clay loam.

A knowledge of the textural grade of a soil suggests numerous
other characteristics of that soil. With experience, even a rough
estimate determined by "feel" is useful, for texture indicates other
physical properties, particularly those affecting moisture, aera-
tion, and workability.

Soil Structure— The arrangement of soil particles becomes es-
pecially important when small size classes are involved. Sands have
single-grain structure, but silts, and more particularly clays, tend
to have particles aggregated in clumps. Aggregation is largely
caused by the colloidal portion, less than 0.001 mm., of the clay.
Just as clay soils with their tremendous internal surface swell when
wet, they also contract as they dry. The minute particles are
drawn together by cohesive forces in large or small aggregates
whose size and shape affect drainage, percolation, erosion, and
aeration (Fig. 79).

If the granular structure is lacking or destroyed by mismanage-
ment, as when trampled by livestock or worked too wet, the soil
puddles or bakes into hard solid masses, and shrinkage results in
the formation of deep cracks. In a loam soil or one with a high
organic content, these undesirable features are reduced while the
desirable characteristics produced by colloids are retained.

Organic Content— The amount of organic material in soil mav
greatly modify its physical characteristics as determined by the
mineral components. In addition, organic material is the major
source of certain plant nutrients, especially nitrogen, so that fer-
tility and productiveness are usually correlated with it.

Under natural conditions, organic matter in soil is derived from
remains of plants and animals. Mostly these remains accumulate
on the surface of the mineral soil to form a layer of litter, which,
if sufficiently thick, may reduce the effects of insolation, check
erosion, and prevent compacting resulting from precipitation.

154 THE STUDY OF PLANT COMMUNITIES * Chapter VII

When decomposition of litter does not exceed accumulation, the
Ao horizon has a surface stratum of undecomposed twigs and
leaves, which is termed the L layer. Beneath this is a stratum of
decomposing but still identifiable plant remains, which is marked
by fungal hyphae in abundance and is called the F or fermenta-
tion layer. In contact with the mineral soil there may be an H or
humus layer if the climate is sufficiently cool and moist. The term,
humus, is applied to material decomposed beyond obvious recog-
nition. Soil animals and percolating water carry the humus into the
soil where, through further decomposition, its chemical constitu-
ents are slowly released for use by succeeding generations of or-
ganisms.

When a distinct layer of humus (H layer) is present with a
rather abrupt transition to mineral soil, the humus type may be
designated as mor. If there is no distinct layer of humus but rather
it is mixed with the surface mineral soil, the humus type is mull.120

Local variations in amount, nature, and rate of decomposition
of humus are to be expected. Evergreen leaves do not decompose
as readily as deciduous ones, nor do they have the same chemical
composition.264 Even the leaves of deciduous species do not all
yield the same decomposition products. Organisms causing decom-
position may be active and abundant in one habitat but quite
incapable of living in another because of such factors as tempera-
ture, moisture, and aeration. Consequently, humus may be un-
equally effective in different habitats, and soils of similar origin
may have quite different productive qualities.

Regional Soil Variations.— Climate, which varies with latitude
and longitude, includes the important factors in soil formation,
especially temperature and rainfall. Within a climatic area, differ-
ences in parent material and topographic position often are re-
flected in soil variations, which may be chemical or physical. Such
variations are most pronounced where parent rock is newly ex-
posed or where soil materials have weathered but slightly, as below
a receding glacier. After longer exposure the developing soils be-
come much more alike, and the longer the time involved, the less
noticeable will be differences related to local conditions. Evidence
is sufficient to indicate that, within a climatic area, soil develop-
ment progresses toward a particular kind of soil and profile regard-

PHYSIOGRAPHIC FACTORS 155

less of the origin or nature of the materials; likewise, that the
ultimate soil group for similar climatic regions will be the same.

Since climatic conditions determine the activities and kinds of
organisms of a region and these organisms in turn contribute to
soil development, it is not surprising that vegetation types and soil
types are closely related. The development of a soil is paralleled
by vegetational changes, the vegetation contributing to soil ma-
turation and the soil controlling the rate of progressive succession
of plant communities, until a mature soil for a given climate sup-
ports a climax community of organisms. Mapping soils on the basis
of mature profile and mapping vegetation on the basis of climax
vegetation should produce closely similar results.

The recognition of climatic soil types originated in Russia. The
approach is well illustrated by Glinka's (1927) grouping of the
great soil groups of the world primarily on a climatic basis. Ac-
ceptance of the idea has become rather general although sometimes
in modified form. The use of specific climatic factors, such as the
relationship between precipitation and evaporation, for delimiting
effective climate produces regions that correspond closely to the
major soil groups.131 In the United States,169, 17° soils are most
often grouped on the basis of mature profiles. Since only the ma-
ture profile is considered, it is a recognition of the same basic ap-
proach used by those determining regional limits through climate,
although it requires that the profile must exist in reality, not as a
potentiality.

Profile Development —Three major processes of soil develop-
ment are concerned in the production of the profiles characteristic
of different climatic conditions.

Podsolization occurs typically in humid, cold temperate regions
where rainfall exceeds evaporation and where vegetation produces
acid humus. The acid decomposition products from the litter in-
crease the solvent power of the plentiful percolating water so that
soluble materials and colloids are almost completely removed from
the surface soil, which is, therefore, of single grain structure at
maturity. Although podsolization occurs under hardwood and
pine forests, its strongest development takes place where spruce,
fir, or hemlock are dominant. The process is partially a product
of the vegetation, for the content of bases in the needles of these

156 THE STUDY OF PLANT COMMUNITIES ' Chapter VII

trees is notably low, and decomposition products of the litter they
produce always give an acid reaction.

Laterization is characteristic of tropical conditions with high
temperatures and abundant rainfall. It is essentially the leaching
of silica from the surface soil. The low acidity produced by de-

.. ..

FlG. 80. The layer of calcium accumulation in a pedocal soil under sage-
brush desert as shown in a road cut in Nevada— Photo by W. D. Billings.

composition of tropical litter promotes the solution of silica as
well as alkaline materials. After laterization, the surface soil is high
in iron and aluminum, which are not removed by the process.

Calcification may occur anywhere but is most important in
regions with low rainfall unevenly distributed throughout the
year and with temperatures producing a relatively high rate of
evaporation. Under these conditions, a permanently dry stratum
may develop in the profile below the depth to which rainwater
penetrates. Carbonates produced by carbonation in the surface
layers, as well as those that may be present in the original soil ma-
terial, are carried downward in solution toward this dry layer.

PHYSIOGRAPHIC FACTORS

157

When the water is removed by plants or evaporation, the carbon-
ates are left behind, at or above the dry layer, depending upon the
depth of penetration of the moisture at the time.

Climatic Soil Types.— On the basis of absence or presence of a
lime carbonate layer formed by calcification, the mature profiles
of all soils of North America fall into two groups : pedalfers, with-
out the layer; pedocals, with the carbonate layer. The two condi-
tions occur regardless of the nature of parent material or its geo-
logical origin, and their distribution is obviously controlled by
climate. Soils of eastern North America are all pedalfers, for the
unfavorable balance between rainfall and evaporation necessary to
carbonate deposition does not occur here. West of about the 99th
meridian (a line through the center of the Dakotas to the pan-

FlG. 81. General distribution of the important zonal soil groups of the
United States. After Kellogg,136 from Klages, Ecological Crop Geography, by
permission of The Macmillan Company, publishers.

handle of Texas), where annual precipitation is normally less than
twenty inches a year, mature profiles almost invariably show
pedocal characteristics except where climatic conditions are vari-
able, notably in the mountains and in parts of California. Climate,
vegetation, and soil have corresponding distributions. The pedal-

158 THE STUDY OF PLANT COMMUNITIES * Chapter VII

fers occur principally in association with forest regions, while the
pedocals do not support forests but are typically covered with
grassland or desert.

Pedalfers.— Although mature soils lying east of the line marking
the western boundary of the prairie are usually of this type, they
vary considerably. The range of temperatures within the area is
so great that podsolization is characteristic in the north and lat-
erization in the south with intermediate conditions represented
between. The following zonal climatic soil groups, therefore,
occur in eastern North America.

Tundra Soils : Far northern soils with shallow profiles and
high proportions of undecomposed organic materials.

Podsol Soils : Northeastern United States and extending
north and northwestward into Canada. Distinct horizons with
a thick Ao, white or gray leached A over a brown B horizon
with its accumulation of aluminum and iron.

Gray -Brown Podsolic Soils : A wide band across east-cen-
tral United States. Like podsol but with thinner Ao horizon
and less leaching of the A, which is grav-brown over a brown
B horizon.

Red and Yellow Soils : Southeastern United States where
humid, warm climate produces both podsolization and later-
ization. Colors bright, low in organic matter, high in clay,
strongly leached. Yellow soils in the sandy, poorly drained
coastal plain; red soils in the well-drained Piedmont.

Prairie Soils .'Western margin of the pedalfers. Intermediate
between forest and grassland soils. Black or dark brown with
brown subsoils that differ little in texture from the surface.

Lateritic and Laterite Soils : Subtropical and tropical. Rep-
resent extreme in mineral weathering. Leached of silica.

Pedocals.— Zonation of these soils from north to south has not
been recognized as for pedalfers. Moisture being more effective
than temperature in producing variation in pedocals, the con-
spicuous zones lie in a north-south position. Their location and
brief characterization follow :

Chernozem Soils : A broad band extending from Canada
into Mexico just west of the Prairie Soils. Rich in organic

PHYSIOGRAPHIC FACTORS 159

matter. Black soils with brown or reddish calcareous subsoils.
Strong carbonate horizon but normal horizons indistinct.

Br oil'?! Soils (also known as Chestnut Soils) : Bordering
Chernozems to the west and developed under successively
drier conditions, they contain successively less organic matter
westward and southward and become lighter in color, as in-
dicated by their division into Dark Brown and Brown Soils.
Occupy mainly the area usually called the Great Plains.

Gray Soils : Desert and semidesert soils largely in the Great
Basin and southward. Gray with yellowish to reddish cal-
careous subsoils. Negligible organic content. Weathering
largely physical.

"Within these climatically determined soil regions, are local varia-
tions that, because of time and topography, bear no resemblance
to the mature soil type. Swamps and bogs, islands and flood plains,
salt and alkali flats, or merely immature soils on steep slopes— all
are illustrations of local conditions that must be disregarded in
considering the broad aspects of climatic control of soil develop-
ment.

The climatic classification of soils is useful because it makes
possible broad considerations of regional problems. It is logical
because it bases the major categories upon mature conditions,
which remain stable with the climate, and makes possible the ex-
planation of local variations, which represent merely stages of
profile development. Best of all, it has world-wide application.
Enough investigations have now been made to show that the same
general soil types are repeated in those parts of the world where
climatic conditions are duplicated. Thus, it has been feasible to
devise several schematic representations of the relationship of tem-
perature and moisture to soil formation that are reasonably ap-
plicable anywhere. A relatively simple climatic system251 is shown
below, in which temperature-evaporation and precipitation-evap-
oration relationships are used as criteria of climatic control. It
serves to emphasize the importance of moisture in pedocal devel-
opment and grassland areas but shows that temperature is more
effective where pedalfers develop with the forests they support.

Vegetation and Soil Development.— The close similarity be-
tween the distribution of major vegetation types and climatic soil

160 THE STUDY OF PLANT COMMUNITIES ■ Chapter VII

DRY COLD

THE DISTRIBUTION OF;

CLIMATIC

TYPES

DRY HOT

2.VEGETATIONAL
FORMATIONS

3. MAJOR ZONAL
SOIL GROUPS

WET COLD

PERPETUAL SNOW AND ICE

TUNDRA

TAIGA

C SUBARCTIC )

1-
Ul

5 CLIMATE

PRECIPITATION

WET HOT

PERPETUAL SNOW AND ICE

TUNDRA

TAIGA

«o

u <
a. (C

Z
<
_l
CO

h
co

10
UJ

a
o

CO •»

l-t-

<0 CL

I
to

UJ w

a.
o

z
<

VEGETATION

PERPETUAL SNOW AND ICE

TUNDRA SOILS

PODSOLS

•0

K
UI

(0
UI

_i
o

o
oc

UJ
IM
O

z
oc

_J
o

CO
UJ
cX
<

a.
Q.

GREY-BROWN
PODSOLIC SOILS

LATERITIC RED
& YELLOW SOILS

LATERITES
RED SOILS

SOILS

FlG. 82. Schematic representation to show the interrelated distribution of
climatic types, vegetational formations, and major zonal soil groups— After
Blumenstock and Thornthwaite.2*

types has been mentioned. It has also been suggested that the char-
acteristics of a mature profile are partially produced by the vege-
tation or that they are possible only because of the kind of vege-

PHYSIOGRAPHIC FACTORS 161

tation supported by that soil in the given climate. This point
should be further emphasized. Newly formed soil material has no
profile and bears no resemblance to the mature soil of the region.
It cannot support the vegetation that grows on a mature soil, but
the plants that can grow upon it contribute to its development,
probably most effectively through their decomposition products,
and so, in time, the resulting soil changes permit other plants to
grow. There results, sometimes over a long period, a succession
of edaphically controlled vegetation types leading ultimately to a
climatically controlled community. Paralleling the plant succes-
sion are changes in the soil— called soil development— -which are
primarily possible because of the plants and which lead to the
mature profile, also controlled by climate. Soil development and
vegetational development are intimately related and together are
controlled by climate.

SOIL WATER*

Soil water probably affects plant growth much more commonly
than any other soil factor. It follows, therefore, that a basic under-
standing of what causes differences in amounts and availability of
soil moisture and what such differences may mean to a plant is
ecologically necessary.

Classification of Soil Water.— A simple, arbitrary system of
classification that divides soil water into four general categories is
sufficient for most ecological purposes.

1. Gravitational water occupies the larger pores of the soil
and drains away under the influence of gravity. For a
short time after a heavy rain or irrigation, the soil may be
completely saturated with water, the air in it having been
displaced from the noncapillary pore spaces between the
particles. Under the influence of gravity, the free water
soon percolates downward through the soil toward the
water table unless prevented by some barrier, such as a
hardpan or other impermeable layer. Within two or three
days after a rain, all the gravitational water usually drains
out of at least the upper horizons of the soil, and the pore

*Much of this section is adapted from a review by Kramer,149 which in-
cludes an extensive bibliography.

162 THE STUDY OF PLANT COMMUNITIES • Chapter VII

spaces become refilled with air. If the soil remains sat-
urated with gravitational water for several days, serious
injury to root systems may result from lack of oxygen
and accumulation of excess carbon dioxide. Hence gravi-
tational water is of little direct value to most plants and
even may be detrimental.

2. Capillary water is held by surface forces as films around
the particles, in angles between them and in capillary
pores. Immediately after gravitational water has drained
away the capillary water is at its peak, and a soil is then
said to be at its field capacity. Much of this film water is
held rather loosely and is readily available to plants, but
some of it, which is held by colloidal material and which
is in the smallest pores, is relatively unavailable. It is in this
connection that the size of particles becomes important. A
cubical sand grain one millimeter on the edge has a surface
of only 6 square millimeters, but if it were divided into
cubes of colloidal size, 0.1 micron on the edge, the total
surface resulting would be 60,000 square millimeters. The
increase in surface and angles between particles would
thus increase tremendously the total capacity for holding
capillary water. However, the water available to plants
does not increase proportionally, for the greater curva-
ture of the films and the sharper angles sufficiently increase
the force with which water is held to materially increase
unavailable water.

3. Hygroscopic water is held in a very thin film on the sur-
face of particles by surface forces and moves only in the
form of vapor. The moisture remaining in air-dry soil is
usually considered as hygroscopic and is, in general, un-
available to plants. Distinction between this and capillary
moisture is difficult, for exposure of soil to increasingly
moist atmospheres may increase the water content even
to saturation.

4. Water vapor occurs in the soil atmosphere and moves
along vapor pressure gradients. It is probably not used
directly by plants.

Origin of Soil Water.— Precipitation in the form of rain, hail, or

PHYSIOGRAPHIC FACTORS 163

snow is the ultimate source of water found in the soil, but not all
precipitation becomes soil water. The steeper a slope, the more
water will run off from its surface before it can enter the soil.
Excessive precipitation in a short period of time results in greater
runoff than that following a gentle rain, since infiltration cannot
keep pace with the rate of fall. If soil becomes saturated and pre-
cipitation continues, little, if any, will enter the soil. A larger pro-
portion of water from slowly melting snow is apt to enter the soil
than from an equal amount of rain. Infiltration into a fine-textured,
clayey soil is slower than into a coarse-textured, sandy soil, and a
compact mineral soil absorbs water more slowly than a loose soil
or one with a high organic content or heavy litter. The particles
of a bare mineral soil tend to pack at the surface when rained upon
for only a few minutes and thus reduce the rate of infiltration (see
Fig. 39). Variation of local conditions may, therefore, modify the
effectiveness of a given amount of precipitation.

Movement of Soil Water.— Water moves downward in quantity
during and immediately after rain or irrigation. Later it may move
upward or laterally to some extent when evaporation and use by
plants reduces the amount near the surface. Its principal movement
occurs as a liquid in capillary films or through noncapillary pores,
but some movement also occurs in the form of vapor. Gravity,
hydrostatic pressure, and capillary action are the forces involved,
and movement may be the result of interaction of all three.

The rate at which infiltration takes place is at first determined
by surface conditions. When they are favorable, practically all of
a light rain is absorbed. Within a half hour or less, however, ab-
sorption declines and is then controlled by conditions in the lower
horizons, where percolation may be very slow. Movement of
gravitational water through the soif is controlled by the number,
size, and continuity of the noncapillary pores through which it
percolates. Drainage is rapid in coarse-textured soils, but in clays
movement is slow since the pores are small and may be blocked by
the swelling of colloidal gels or by trapped air. Channels left by
earthworms or other animals and those left by dead roots greatly
facilitate downward movement. If there is no impermeable hard-
pan layer and if the water table has not been raised too near the
surface, all gravitational water drains from surface strata within

164 THE STUDY OF PLANT COMMUNITIES • Chapter VII

two or three days after a rain leaving the soil water content at
field capacity.

A simple explanation of the movement of capillary water may
be entirely adequate for most ecological purposes. Since capillary
water forms a continuous, thin film around soil particles and in
the small spaces and angles between them, it is obvious that sur-
face tension of the water creates inward pressure in the film and
that water, therefore, tends to move from regions with thicker
films to regions with thinner films. An explanation with broader
applications considers- the difference in attraction for water be-
tween two portions of soil having different moisture contents and
expresses this attraction or force as capillary potential— -that is, the
force required to move a unit mass of water from a unit mass
of soil. Various methods of measuring this force indicate that the
potential is directly related to the water content and that there is
no change in the state of water as moisture content is reduced
from field capacity to an oven-dry condition, but merely an in-
crease in energy required to move it. On this basis, the boundaries
between gravitational, capillary, and hygroscopic water are too
indistinct to be recognized. That these boundaries are indistinct is,
in fact, true regardless of the point of view. Such relatively simple
considerations seem entirely satisfactory for an adequate under-
standing of plant-water relationships, although recent studies of
soil moisture by soil physicists have become increasingly technical.

Movement of capillary water is closely related to soil texture. In
wet soils, it is rapid in sand and slow in clay, but the rate is re-
versed as soils dry out. Capillary rise, or the distance that capillary
force will move water, is much greater in clay than in sand al-
though the rate of movement is less in clay. The rate is surpris-
ingly slow at all times and probably is quite insufficient to main-
tain an adequate film on the soil particles from which a root is
removing water. The water coming to a root by capillary action
does not at all equal the amount made available in new films that
the root contacts because of its elongation and production of new
root hairs. When soil water is below field capacity, capillary move-
ment is probably insufficient to replace the film on particles from
which roots of an actively transpiring plant are removing water.
The continuous elongation of these roots with the production of

PHYSIOGRAPHIC FACTORS 165

new root hairs brings them in contact with new films and helps
to keep up the supply of necessary available water.

Movement of water vapor is along vapor pressure gradients,
which are affected by temperatures and vapor pressures of the air
and the different soil horizons. There must, therefore, be some
movement in all soils, but its effects are most noticeable in semi-
arid regions where there is no connection between the water table
and capillary water near the surface. In winter or in any cool pe-
riod, water vapor moves upward from the warmer subsoil and
cools and condenses in the surface layers. When temperatures rise
at the surface, evaporation takes place into the air, and the total
ground water is reduced. Usually the surface soil is warmest in
summer and results in downward movement of vapor with con-
densation at lower levels. If the surface soil is cooler than the air
above it, water vapor may move into the soil and condense there
in quantities sufficient to be of significance under semiarid condi-
tions.

Water Lost to the Atmosphere.— The loss of water from soil to
the air by evaporation varies with the factors affecting the steep-
ness of the vapor pressure gradient. Temperature, humidity, and
movement of the air, as well as temperature and moisture content
of the soil, are factors, which in turn are modified by exposure,
cover, and color of the soil. Probably the loss of water by evapora-
tion is much less than is commonly supposed, for numerous studies
indicate that there is little capillary rise to replace water lost by
evaporation unless the water table is within a few feet of the sur-
face. In those areas where water lost by evaporation might be
critical, the water table lies so deep that precipitation rarely wets
the soil down to it and, consequently, the upward rise is of no
consequence. In general, the loss of water by evaporation seems
mostly to be from the top foot of soil. Under natural conditions,
this probably affects few species and is rarely of significance.

In agriculture, water lost by evaporation has been the subject of
much argument, particularly with regard to the effects of cultiva-
tion. Evaporation from a dry soil surface is much less than from a
moist one because diffusion through soil is very slow. Since a dry
soil surface can be moistened only by an upward capillary move-
ment of water if no rain falls, it has been maintained that cultiva-

166 THE STUDY OF PLANT COMMUNITIES ' Chapter VII

tion of the surface must reduce loss by evaporation since it pre-
vents capillary movement. It is now known that, unless the water
table is very near the surface, capillary rise is negligible under any
circumstances. This being true, the dust mulch, or cultivated sur-
face, has little to support it. In fact, if the surface capillary water
is not connected with the water table, as is frequently true under
irrigation, cultivation for a mulch probably increases the loss of
water. Organic mulches seem to be more effective in reducing
water loss, probably because they shade the soil and reduce its
temperatures, increase the distance of diffusion from soil to air,
and protect the soil from the drying effects of wind.

Water lost to the atmosphere through transpiration far exceeds
that lost by evaporation. Whereas evaporation seems to be effective
only in the surface soil, plants remove water from considerable
depths. Studies of orchard soils in different parts of the country
indicate that all readily available water may be removed to a depth
of three to six feet in three to six weeks, depending upon atmos-
pheric conditions and the kind of soil. Sandy soils, of course, are
exhausted more quickly than clayey soils. The relative losses by
evaporation and transpiration are illustrated by experiments,262 in
which water was lost from a bare soil surface in a tank at the rate
of 4.7 pounds per square foot during one growing season, while a
four-year-old prune tree removed water from a similar tank at the
rate of 416 pounds per square foot of soil surface. An acre of de-
ciduous fruit near Davis, California, used eight acre-inches of
water in six weeks in midsummer. Corn grown in Kansas requires
some fifty-four gallons of water per plant to mature. If this were
applied at one time, as by irrigation, it would cover a cornfield to
a depth of about twelve to fifteen inches. Plants growing natural-
ly have similar requirements. The knowledge that transpiration is
the chief means of reducing capillary water in the soil has led to a
consideration142 of what kinds of plants on watersheds will least
reduce the supply of water by transpiration and still prevent ero-
sion.

Soil Moisture Constants.— To compare the moisture character-
istics of soils or to discuss them with respect to plants, quantita-
tive expressions of hydro-physical properties are a necessity. These
properties, determined under fixed conditions, are called constants.

PHYSIOGRAPHIC FACTORS

167

The hygroscopic coefficient is the moisture content, expressed
as a percentage of the dry weight, of a soil in equilibrium with an
atmosphere of known relative humidity. The value is difficult to
obtain with accuracy and is of little use to plant scientists.

\\^' —Inner Cylinder

— Cutting Cylinder

062

FIG. 83. Sampler for obtaining undisturbed soil for determining volume-
weight, air space, and water holding capacity. A counter-sunk steel plate or
a block of wood placed on the cylinder prevents it from being battered
when driven into the soil with a sledge hammer. The inner cylinder (see Fig.
79) is removed with the sample (600 cc.) and is covered with tightly fitted
lids for transportation.— After Coiled

168 THE STUDY OF PLANT COMMUNITIES • Chapter VII

Maximum "water holding capacity is the water held by a sat-
urated soil. It may be determined by weighing a unit volume of
soil before and after it has been immersed in water for twenty-four
to forty-eight hours.

Field capacity is the amount of water a soil retains after all
gravitational water is drained away. Soils in the field attain this
condition within one to five days after a rain except when the
water table is near the surface or saturation extends to a depth of
many feet. After prolonged rain, soil may be assumed to be at field
capacity if samples taken at eight- to twelve-hour intervals have
essentially the same moisture content.

It is now common practice to express most soil moisture values
on a volume basis. In addition, it is desirable that most of these
values should apply to the soil as it lies in the field. It is, therefore,
advisable to obtain undisturbed samples of a certain volume and to
make all determinations without modifying the structure of the
samples. Such samples may be obtained with metal cylinders,63
which, when forced into the soil, cut a sample of exact volume,
which is then enclosed with airtight lids. Rocky soils may make it
impossible to obtain undisturbed samples. It then becomes neces-
sary to use special techniques, which, although they give much
the same results, require more time and pains than are ordinarily
necessary.163 Some investigators obtain all their samples only when
the soil is at field capacity. This system has several advantages,
such as eliminating the problems related to swelling on wetting,
simplifying sampling, and giving a value for field capacity that is
strictly determined by field conditions. When soils are dry, it is
often possible to soak them, in place, and permit them to come to
field capacity before sampling.

Capillary capacity (water holding capacity) is the water re-
tained against the pull of gravity. Although this appears to be
essentially what is meant by field capacity, it is a value determined
under laboratory conditions and may run slightly higher than field
capacity. The saturated samples of undisturbed soil used for de-
termining maximum water holding capacity are permitted to drain
over sand for a fixed time, usually two hours, and the weight of
water retained, expressed as a percentage of the volume of the
sample, is termed the capillary capacity.

PHYSIOGRAPHIC FACTORS 169

When the maximum water holding capacity, the field capacity,
or capillary capacity, and the dry weight of an undisturbed sam-
ple are known, it is relatively simple to calculate pore volume, air
capacity, volume weight, and specific gravity of soil material.162

The moisture equivalent denotes the water content of soil that
has been subjected, usually for thirty minutes, to a centrifugal
force of one thousand times gravity in a soil centrifuge. Its deter-
mination is simple if equipment is available. Within limits, it bears
a constant relationship to certain other soil moisture values or, at
least, suggests what these values should be. Its ratio to field ca-
pacity is near unity, but the relationship is least constant with
coarse-textured soils. In many soils the moisture equivalent is 1.84
times as great as the water left in those soils when plants wilt. Un-
available water can, therefore, be approximated from the moisture
equivalent. The ratio of moisture content to moisture equivalent
(relative wetness) can be used to make comparisons between soils
or soil strata of different textures where moisture content alone
would mean little in terms of plants because of variation in avail-
ability.

The permanent wilting percentage should be considered as the
moisture content of the soil at the time when the leaves of plants
growing in that soil first become permanently wilted. Because it
has not always been so considered, there have been various other
terms (wilting point, wilting coefficient, wilting percentage) ap-
plied to the concept, and not all investigations have produced the
same results. Briggs and Shantz35 first emphasized the importance
of this soil moisture condition to plant growth and called it the
"wilting coefficient!' Their procedure was to grow seedlings in
glass tumblers of soil sealed with a mixture of paraffin and vase-
line. When the leaves wilted and did not recover overnight in a
moist chamber, the moisture content of the soil was determined
by oven drying at 105° C. and calculated as a percentage of the
dry weight. It is generally agreed that permanent wilting marks
the soil water content at which absorption becomes too slow to
replace water lost by transpiration.

Briggs and Shantz came to the conclusion that soil texture alone
determines moisture content at which plants wilt permanently,
regardless of the species, their condition, or the environmental

170 THE STUDY OF PLANT COMMUNITIES * Chapter VII

conditions. This conclusion was not immediately acceptable to
everyone, and numerous studies were made to check its validity
with different kinds of plants of different ages under a variety of
conditions. It is now generally agreed that permanent wilting of
any species occurs at the same water content of a soil of a certain
texture regardless of the age of the plant or environmental condi-
tions under which it grew. Uniformity of results is assured if non-
cutinized herbaceous plants are used and if permanent wilting of
the lowest pair of leaves is used as an end point. This eliminates the
problem of recognizing the onset of permanent wilting and varia-
tions related to the ability of some plants to live much longer than
others after the onset of wilting.

Briggs and Shantz also concluded that it was possible to calcu-
late the wilting point from the moisture equivalent because the
following relationship held in their soils :

... ~ . moisture equivalent

wilting coefficient == 3

5 1.84 ±0.013

Although this often holds true, it does not apply to all soils. Studies
in different parts of the country indicate that the ratio ranges at
least from 1.4 to 5.65. Attempts to relate the moisture content at
the time of wilting to other variables have been equally unsatis-
factory, and it, therefore, appears that its determination is most
reliable when observed directly. Because the expression, "wilting
coefficient" has been so often associated with calculated values, it
is logical to restrict it to that usage and to apply the term, "per-
manent wilting percentage'/ to determinations made by direct ob-
servation.

Readily available water is that which can be used by plants for
growth and is, therefore, the moisture above the permanent wilt-
ing percentage. This includes gravitational water, but its rapid
drainage makes it of little consequence. The remaining usable
water is in the range from field capacity or moisture equivalent
down to the permanent wilting percentage. This range is narrow
in sand and wide in clay. Obviously, the wider the range, the
longer plants can resist drought and, in cultivation, the less fre-
quently irrigation is necessary. The rate at which water moves
from soil to an absorbing surface is strongly indicative of plant-
soil moisture relationships at the time. An indication of the avail-

PHYSIOGRAPHIC FACTORS 171

ability of soil water to the plant may be obtained with porous soil
point cones,158 whose rate of absorption is taken as the basis for
evaluating water supplying povcer of the soil.

Availability of Soil Moisture to Plants.— Gravitational water is
readily available to plants only when present in a saturated soil, a
condition that rarely continues long enough to be of importance.
Normally, then, readily available water is that capillary water in
the range between field capacity and the permanent wilting per-
centage. It is usually lowest in sand, and highest in clay. The fol-
lowing values for readily available water are found in some North
Carolina soils :149 sand, 2 percent, sandy loam, 14 percent, clay, 19
percent. However, this generalization does not always hold, for
some clays may have high field capacities but also have high wilt-
ing percentages. A California clay with a moisture equivalent of
31 percent was found to have a wilting percentage of 25 percent,
and, therefore, it could contain only 6 percent of available water.
Such a soil would store less water for plant use than many sandy
soils, and plants growing in it would suffer from drought much
sooner than its soil texture would indicate. This also explains why,
in contrast to the usual situation, sand dunes in deserts have more
favorable moisture conditions than the surrounding clay soils.
When both are at or near the wilting percentage, as they fre-
quently are, a typically light rain provides little or no available
water in the clay but does provide some in sand, in addition to
penetrating more deeply, because of the lower wilting percentage
of sand.

Whether or not all available water is equally available to plants
is not entirely agreed upon. The evidence from a variety of
sources seems to favor a decreasing availability as the supply is
reduced toward the permanent wilting percentage and particu-
larly in the lower half of the range of available water. Another
factor affecting the availability of soil water is the concentration
of the soil solution, which, if high, may have a toxic effect on
plants and also modify their osmotic activity. Soil temperature,
too, may be effective. Water supplying power may be reduced by
half when soil temperature is lowered from 77° E to 32° F. Prob-
ably the increase in viscosity of water at low temperatures reduces
the rate of movement from soil to absorbing surface.

172 THE STUDY OF PLANT COMMUNITIES • Chapter VII

Measurement of Soil Moisture.— For ecological purposes, it is
of prime importance to know how much soil water is available for
plant use and often to be able to follow its variations from day to
day throughout a growing season. Because of soil variation, it is
usually desirable to have determinations from numerous places in
a stand and usually from more than one stratum in the soil. It is
undesirable to use sampling methods that disturb any considerable
amount of the soil or injure roots in the experimental area, and,
again, any expression of soil moisture should preferably refer to a
unit volume of sample obtained in an undisturbed condition. The
last qualification is advisable because interest is in the volume of
water available to roots occupying a given volume of soil, rather
than weight of water in a given weight of soil.

It should be clear from our previous discussion that, to inter-
pret soil moisture conditions, several soil moisture constants are
necessary and that some physical analyses of the soil may be de-
sirable. A single collection of samples from each local area of study
may suffice for these purposes. Thereafter, some method must be
fixed upon, which, within the time available to the worker, will
give as adequate a notion as possible of the variations in soil mois-
ture content of the experimental areas. Finally, it must be possible
to express the soil moisture data in terms of what is available to
plants.

Methods currently in general use are of two types : ( 1 ) deter-
mining the actual content of water, (2) measuring the forces with
which water is held or the rate at which it is supplied to an ab-
sorbing surface.

The actual content of water is determined by weighing samples
before and after drying to constant weight in an oven at 105° C.
The loss in weight, representing the water content, is expressed as
a percentage of the dry weight or, if the samples are undisturbed,
on a volume basis. The disadvantages of the method are numerous.
Sampling takes time and disturbs the soil, the samples must be
transported, weighing and drying are time-consuming, and a con-
tinuous record is impossible. However, the method has its uses,
and, where only a few determinations are wanted, it is undoubt-
edly the procedure to use. Note, too, that it requires no equip-
ment that is not ordinarily available.

PHYSIOGRAPHIC FACTORS

173

Several electrometric methods have been adapted to the meas-
urement of soil moisture. All require calibration in terms of the
wilting percentage of the soil involved but thereafter permit rapid
determinations at short or long intervals and direct translation of
measurements into available water. The method that seems to be
most in favor at present is the measurement of resistance between
two electrodes imbedded in gypsum blocks and buried in the
soil.29 The resistance varies inversely with amount of soil water
and also with soil temperature. Other methods measure dielectric
constant, or electrical or thermal capacity of the soil, values that
vary with changes in soil moisture.

Two physical measurements, making use of (1) tensiometers207
and (2) soil point cones,158 have been used successfully.

A tensiometer measures the tension existing between the soil
and the soil water. It consists of a porous cup set in the soil and
connected to a manometer by a tube of small diameter. Water in
the instrument makes connection through the porous cup with the
soil water, from which equilibrium tension is transmitted to the
mercury of the manometer. Since the height of the mercury col-
umn indicates the tension in the soil, the manometer can be cali-
brated for a wide range of soil moisture values, and readings can
be taken at any time and translated directly into values for avail-

TABLE 9— Percentage composition of oxygen and carbon dioxide in soil
air extracted at different depths in a silty loam soiK30). Note that the percent-
age of 02 decreases and of CO2 increases with depth both winter and sum-
mer but that subsoil aeration is far better when the soil is dry in summer than
when it is wet in winter.

Depth

Oxygen

Carbon dioxide

(feet)

Winter

Summer

Winter

Summer

19.4

19.0

1.2

2.4

11.6

17.4

2.4

3.7

3.5

16.7

6.6

5.0

0.7

15.25

9.6

8.55

2.4

12.95

10.4

11.85

0.2

11.85

15.5

11.9

174 THE STUDY OF PLANT COMMUNITIES * Chapter VII

able water. The instrument is accurate for values ranging from
zero tension to approximately 0.85 atmosphere of tension, or from
saturated soil to a reduction of 80 or 90 percent of available
water.217 Approaching the wilting percentage, its values cannot be
wholly trusted.

Soil point cones are small, hollow cones of porous porcelain,
which can be inserted into the soil with a minimum of disturbance
so that each has an equal area of surface in contact with the soil.
The amount of water absorbed by the cone is determined by
weighing and is taken as a measure of the water supplying power
of the soil. In some types of studies, this value alone is sufficient to
make comparisons between soils without any further analyses be-
ing necessary. It is also indicative of moisture conditions, for, at
the wilting percentage, it approximates 0.085 g. in two hours.

SOIL ATMOSPHERE

Organisms and Soil Atmosphere.— It was pointed out earlier
that air is a component of soil (p. 148). Both the amount and com-
position of this air are of importance to plants. Most plants re-
quire a well-aerated soil for growth and even for survival. Many
seeds will not germinate unless well aerated even though tempera-
ture and moisture are favorable. Healthy roots must carry on res-
piration continuously, which means that oxygen must be present
in the soil. At the same time, their activity produces carbon dioxide
and carbonic acid, which tend to accumulate. To some plants,
the increase in the proportion of carbon dioxide is more injurious
than the decrease of oxygen. Since all microorganisms present are
likewise using oxygen and releasing carbon dioxide, the balance of
the two cannot be maintained unless there is a free exchange of
gases with the air above the soil. If aeration is good, this may be
accomplished by diffusion from the air. However, in any soil the
proportion of oxygen decreases and that of carbon dioxide in-
creases with depth, and the proportion of oxygen is not as great
in soil as in air even when conditions are most favorable.

Relation to Growth and Distribution of Roots.— Since aeration
becomes poorer and oxygen decreases with depth of soil, these
conditions may limit the depths to which roots can grow. The
deepest root penetration is in well-aerated soils. Species growing

PHYSIOGRAPHIC FACTORS

175

in wet lowlands are invariably shallow-rooted, for here aeration is
poorest because the soil is periodically or continuously saturated
and the only available oxygen may then be in solution. These
shallow-rooted species will usually grow well in uplands, but, if
the naturally deep-rooted species are moved to lowlands, they do
not do well or may actually die. Thus aeration may determine the
rate of growth, an element of importance in forest stands, and may
be the factor controlling the type of vegetation.

Soil Aeration and Plant Adaptations.-Well-aerated soils may
have an air capacity of 60 to 70 percent by volume, a condition
determined primarily by structure and scarcely affected by tex-
ture. The amount of air varies, of course, with the water content
of the soil, for air is forced from the spaces in the soil that become
occupied by water.

Thus continuously saturated soil is poorly aerated, and the mud
under a pond probably has the poorest aeration of any plant habi-

FlG. 84. Some types of lacunar tissue found in stems of emergent and
other aquatic vascular plants. (A) Cortex of water milfoil (Myriophyllum).
(B) Ground parenchyma throughout stem of a rush (J uncus). (C) Same for
a sedge (Cyperus).

176 THE STUDY OF PLANT COMMUNITIES * Chapter VII

tat. Most species growing in such habitats have adaptations that
serve to counteract poor aeration. Many have large, continuous
spaces— lacunar tissue— in their stems and roots permitting storage
and free movement of gases within the plant. In emergent and
floating-leaved species, these spaces are connected directly with
the atmosphere through the stomata. Submerged leaves of aquatics
are invariably finely dissected or extremely delicate, conditions

Fig. 85. Cypress swamp (Taxodium distichum) in the coastal plain of
South Carolina. Note buttressed, somewhat planked bases of trees and an
abundance of cypress knees, whose uniform height marks average high-water
level.— U. S. Forest Service.

that bring a majority of the cells in contact with the water, from
which they must obtain oxygen in dissolved form. A few sub-
merged species produce pneumatophores, or special branches that
extend above water and give direct connection with the air through
lacunar tissues. In addition to shallow root systems, a number of
swamp trees have other characteristics in common. Enlarged or
buttressed bases and plank roots are frequent, especially in south-
ern swamps, and the "knees" of cypress are in the same category.
That these structures facilitate aeration has not been conclusively
demonstrated, but their formation seems to be in response to alter-
nate inundation and exposure to air.150

PHYSIOGRAPHIC FACTORS

177

Determination of Volume and Composition.— Total pore space
or pore volume (in c.c.) is equivalent to the weight of water (ing.)
in the soil at saturation, for water then is assumed to occupy all
the space in the soil. Actually not all air can be replaced by water,
and the small amount of air remaining at saturation represents
what is available to roots regardless of circumstances. Air capacity
is the amount of air in soil that has been drained of all gravitational
water. It is, therefore, equal to the difference between pore vol-
ume and the weight of water at field capacity. Since total water
holding capacity and field capacity are constants, it follows that
pore volume and air capacity are soil-air constants. The actual air
content is not at all constant, for it varies inversely with the water
content. Soils with a high air capacity are in general well aerated,
but, after prolonged rain or flooding, they may for a time be
poorly aerated because water fills so much of their internal space
that actual air content is low.

TABLE 10— Porosity, field capacity, and air capacity of some soils with
different textures. After14 from Kopecky.

Character of soil

Compact heavy clay . . . .

Clay loam

Compact loam

Very fine sand

Friable loam

Friable fine sandy loam

Particles

smaller

Total

than

pore

Field

o.oi mm.

volume

capacity

86.7%

47.6%

48.0%

67.2

41.1

46,1

46.5

34.9

41.1

48.4

39.3

49.3

42.6

37.1

49.3

39.6^

34.6

49.5

Air
capacity

0.4%

5.0

6.2
10.0
12.2
14.9

The composition of soil air may be determined with a portable
gas-analysis apparatus.113 The sample of air can be pumped from
the soil through a sampling tube211 or some similar device,30 or it
can be withdrawn from a unit volume of undisturbed soil. The
total percentage of oxygen and carbon dioxide in the soil is usu-
ally very nearly that found in air, and, in general, an increase of
one results in a proportional decrease of the other.

178 THE STUDY OF PLANT COMMUNITIES * Chapter VII

SOME CHEMICAL FACTORS

Soil Acidity.— Regardless of the nature of their parent material,
soils tend to become acid in reaction if precipitation is sufficient
to cause downward percolation of water during much of the year.
This is largely the result of leaching of soluble basic salts. To illus-
trate, calcium carbonate is relatively insoluble in water but reacts
with carbonic acid, ever-present in soil water, to form readily
soluble calcium bicarbonate. This, of course, is leached from the
surface soil by percolating water. Although the leaching bicar-
bonates may be re-precipitated at any time the soil dries out, they,
nevertheless, tend always to move downward. Thus the surface
horizons tend to be low in basic materials and may have a highly
acid reaction because of the acids produced by chemical and
biological activity in progress there. The surface strata have the
largest accumulation of organic matter, which yields acid prod-
ucts upon decomposition, the greatest numbers of soil organisms
whose activities may produce acids, and the most active chemical
changes in the mineral components, also contributing to acidity.
Consequently, acidity is normally greatest at the surface and de-
creases in the lower horizons of the soil.

A solution is acid when the concentration of hydrogen ions
(H+) exceeds that of hydroxyl ions (OH), and it is alkaline if
there are more OH~ ions than H+ ions. If the two concentrations
are equal, as in pure distilled water, the reaction is neutral. Since
the concentration at neutrality is known, an expression of the H+
ion concentration in a solution indicates its degree of either acidity
or alkalinity.

Because H+ ion concentration involves numbers too cumber-
some for ordinary use, negative logarithms of the numbers are
substituted and preceded by the expression pH. Neutrality is ex-
pressed as pH 7.0, indicating a solution that is 0.0000001 (or 10~7)
normal in H ions. A pH value below 7 indicates a greater concen-
tration of H ions, or acidity, and a value larger than 7 indicates
alkalinity. Since the pH values are logarithmic, the relationships
between them are geometric and acidities at pH 5.0, 4.0 and 3.0
are respectively 10, 100, and 1000 times as great as at pH 6.0. The
pH of most soils will normally fall between 3.0 and 9.0, and, in hu-
mid regions, the range to be expected is considerably less, perhaps
no greater than pH 4.0 to 7.5.

PHYSIOGRAPHIC FACTORS 179

Under ordinary conditions, the hydrogen ions themselves prob-
ably have little direct effect upon plants, but degree of acidity of
the soil may have a regulatory effect upon chemical processes that
do influence growth. Increased acidity may reduce availability of
nutrients, as when phosphorous combines with aluminum and iron
to form insoluble phosphates. High acidity may, apparently, pro-
duce toxic effects, but these are not caused by H ions. It is more
likely that they result from soluble compounds of aluminum and
iron, which form in increasing amounts as the H ion concentration
rises. Since lime is a required nutrient, its characteristically low
content in acid soils may be of more importance than the degree
of acidity. Numerous soil organisms are sensitive to changes of
acidity, and, if their activities are inhibited, decomposition of or-
ganic matter may be retarded, nutrients may not be released, and
nitrification and nitrogen-fixation may be checked.

With such a variety of things that may be affected by soil
acidity, it should be suspected that a simple relationship between
pH and plant responses does not exist. Studies of soil pH and plant
distribution bear this out, for, in general, if the environment is
favorable and necessary nutrients are available, most species can
tolerate a rather wide range of pH. At the same time, many of
these species reach their best development or are most abundant
within a restricted portion of that range of pH. It should be clear
that, even under such conditions, pH alone cannot be the limiting
factor.

Determinations of pH may be made colorimetrically by the use
of indicator solutions or electrometrically with a potentiometer
and a glass electrode.206 A very useful approximation may be made
with a universal indicator, which, when placed in the soil solution,
takes on a color corresponding to a particular pH value. This is
handy in the field since it requires no more than a small bottle of
indicator and a pocket-size porcelain plate on which permanent
color standards are painted. More accurate colorimetric deter-
minations require a series of indicators whose colors correspond
to overlapping pH ranges. When electrometric equipment is avail-
able, it is preferable because of its accuracy.

Exchangeable Bases.— Ecologists have given relatively little at-
tention to the ways in which the mineral nutrients of the soil

180 THE STUDY OF PLANT COMMUNITIES * Chapter VII

affect plant distribution and growth of wild species. An important
part of the mineral nutrition of native and cultivated vegetation is
derived from the exchangeable bases or cations adsorbed on the
surfaces of the soil colloids. When these vary considerably in
amount or kind, there may be marked differences in the type of
vegetation or at least in rate of growth. For example, it has been
shown that, in soils derived from hydrothermally altered rocks in
the Great Basin, sagebrush and its associated species fail to grow
because of the very low percentage of exchangeable bases as com-
pared with the normal brown soils of the sagebrush zone.22

The colloidal portion of the soil is composed primarily of alum-
ino-silicates. These colloidal particles are almost always negatively
charged, and upon their surfaces are adsorbed great numbers of
cations. These cations are principally H+, Ca++, Mg++, K+, and Na+,
named in the decreasing order of tenacity with which the cations
are held. The hydrogen ion is held more tightly than calcium and
replaces calcium more readily than calcium will replace hydrogen.
This same relationship holds between calcium and magnesium, and
so on down the series. The displaced cation usually enters the soil
solution. This phenomenon, in which one cation may replace an-
other on the colloidal particle, is called base exchange.

Plants are almost entirely dependent on this process of base ex-
change for their supply of calcium, magnesium, and potassium. Of
the anions, only POi is held to any extent by colloidal adsorp-
tion, the other anions, such as NO.r, being readily soluble in the
soil solution and therefore, readily leached. One source of the H
ions that can displace the bases and make them available is the car-
bonic acid formed when carbon dioxide from root respiration is
released into the soil solution. This was shown experimentally for
the calcium ion.132 Another common source is the organic acids
derived from humus.

Soils differ widely in their ability to supply cations because of
the effects of climate, parent material, and vegetation. The maxi-
mum amount of exchangeable cations a soil can hold is called
the base exchange capacity of the soil. Obviously, a soil high in
colloids will have a high capacity as compared with one low in
colloids, as, for example, a sand. Even the kind of clay may make
a great difference in the base exchange capacity of a soil. For ex-

PHYSIOGRAPHIC FACTORS

181

ample, kaolinite has a very low capacity compared to clays of the
montmorillonite group, which have relatively high capacities.

Since soils are constantly losing some of their adsorbed bases
due to replacement by H ions, the soil is rarely, if ever, saturated
with bases to its capacity. The degree of saturation at any given
time is known as the percentage of base saturation of the soil. The
base exchange capacity of a soil minus the percentage of base sat-
uration is theoretically equivalent to the percentage hydrogen
saturation of the soil, since hydrogen is the replacing ion.

Both climate and vegetation have great effects upon the amounts
of exchangeable bases present in soils. On soils derived from the
same parent material, sugar maple-beech-yellow birch forest
maintains a soil at a higher percentage of base saturation than that
under a red spruce forest.53 This seems to be due largely to the
ability of the hardwoods to absorb calcium from the subsoil and to
add it to the surface soil by leaf fall.

Ca Ca K Ca K

Ca Mg Ca K

Ca H Ca Mg

H H

Colloid

H Mg

H
H
H

K Mg Ca Na Ca

Ca Na

Ca H Ca

Arid region

Desert soils

Arid brown soils

Chestnut soils

Transition zone
Chernozems

Humid region

Gray-brown-

podsolic soils

Podsols

Many investigators have shown the relation between precipita-
tion, percentage base saturation, and pH. In brief, it may be stated
that, in regions of high precipitation, the bases are readily replaced
by hydrogen ions and then leached from the soil. The excess of
hydrogen ions results in lowering the pH and creating an acid soil.
Such conditions prevail in the cool, moist, coniferous forests of
the north. Just the opposite conditions prevail in the soils of arid
regions where low precipitation and scanty vegetation combine to
allow the bases to remain on the colloids, thus maintaining a hio-h

182 THE STUDY OF PLANT COMMUNITIES * Chapter VII

percentage saturation and pH. These relationships are represented
schematically on page 181.131

Inhibition of Growth by Plant Products.— That certain plants
produce soil conditions inhibiting the growth of other plants is
probably true.273 Over a hundred years ago it was argued that crop
rotation was necessary for this reason and that fallowing of land
favored the next crop because it permitted the leaching of harmful
excretions or by-products of decomposition resulting from the
previous crops. Today we cannot entirely ignore this line of rea-
soning, for explanations of the benefits of rotation and fallowing
based upon nutrient deficiencies are not always adequate. Like-
wise, there is some evidence that toxic substances are released in
the soil as excretions,215 or when external root cells are sloughed
and decompose,209 or when the plants disintegrate after death.

A number of grasses inhibit growth of other plants. In lawns,
certain strains of bluegrass almost completely check the growth of
white clover.1 Walnut inhibits the growth of a number of herbs.
Fairy rings of both fungi and higher plants may be the result of
toxic products produced by the plants, for other explanations do
not always suffice. If water, supplied in excess to flats of experi-
mental plants, is permitted to percolate through the soil and is then
used as the water supply for other plants, the latter are frequently
inhibited in growth even under the most favorable conditions.17
Extracts from decomposing plant remains have produced similar
results. Apparently toxic or growth-inhibiting substances are pro-
duced by a number of plants, which may affect germination of
seeds and growth of seedlings, or even of mature plants of the
same or other species. Some species are affected, others are not.
Whether higher plants are affected directly is not always clear.
Perhaps effects upon soil organisms and their activity in turn af-
fect the higher plants.

The subject is controversial, and some evidence is conflicting.
The limited information that is available is often derived from
observation of agricultural soils and cultivated plants. Cultivation,
probably because of better aeration, reduces the effectiveness of
inhibiting substances, and the problem is practically eliminated by
crop rotation and the compensating effect of fertilizer. It is, there-
fore, not surprising that investigators have turned to other things.

PHYSIOGRAPHIC FACTORS 183

In natural soil, however, these artificial modifications are absent,
and, consequently, in view of the possible implications in inter-
preting associations of species or the causes of succession, it is sur-
prising that the subject has not been given more attention.

Alkalinity.— Soils with an alkaline reaction have usually orig-
inated from limestone, dolomite, or marble in which calcium car-
bonate is the basic mineral. The CaCOa tends to neutralize acids
that appear in the soil, and the degree of alkalinity is proportional
to the solubility of the limestone. Dolomite contains more MgCOs
than CaC03, and gypsum is largely CaSCh, but the soils they form
contain CaC03, and their floras are essentially similar to that of
limestone. In our arid West, soils are often alkaline in reaction be-
cause of the sodium ions, which accumulate as sodium hydroxide
(NaOH).

Neutral or alkaline soils favor the activities of most soil organ-
isms and the availability of nutrients for higher plants. At the same
time, the tendency of soil colloids to aggregate and produce crumb
structure in the presence of lime results in soil structure with
water, air, and temperature conditions favorable to plant growth.
Thus most cultivated crops do best on soils with a pH ranging
close to neutrality. Native plants, in general, respond similarly, but
there are exceptions, which require, on the one hand, high concen-
trations of CaC03 or, on the other, extremely acid conditions re-
gardless of other factors.

Not all species found growing in calcareous soils are calciphiles.
The distribution and occurrence of many show no correlation
with alkalinity of the soil. A considerable number of these widely
distributed species may, however, grow more luxuriantly when on
calcareous soil. Some, although not restricted to the habitat, will
be found there characteristically. These are true calciphiles. There
are, in addition, obligate calciphiles, which grow only in cal-
careous habitats.

The exceptional vigor on calcareous soils of otherwise wide-
spread species may result simply from the improved aeration,
moisture, or nutrient conditions produced by lime. Calciphiles
may grow on other than alkaline soils if competition from non-
calciphiles is not too great. The less favorable are the general con-
ditions for growth, the more the calciphiles are restricted to their

184 THE STUDY OF PLANT COMMUNITIES * Chapter VII

alkaline habitat, and, as a result, at or near the limits of their ranges
they often appear as obligate calciphiles.

Salinity.— Under conditions of poor drainage and high tempera-
ture, much of the water deposited in low places evaporates and
leaves behind the salts it has carried from the soil of surrounding
slopes. If precipitation is seasonal and alternates with extreme
drought, there is insufficient leaching to prevent accumulation of
these soluble salts, which then form alkali soils, so called regard-
less of the salt involved. Alkali soils of various kinds occur in all
parts of the world and are common in the arid portions of western
North America. Lowlands bordering the oceans are subject to
periodic inundation with sea water and, consequently, contain
relatively high concentrations of salts.

Plants that can tolerate the concentrations of salts found in
saline soils are termed halophytes. How they survive where ordi-
nary plants have little chance has been the subject of much debate.
If not actually dry, these saline habitats may be termed "physi-
ologically dry" because of the high concentrations of salts, which
would limit osmotic activity and, consequently, absorption of
water by the ordinary plant. The morphological and anatomical
characteristics usually appearing in plants of arid regions are com-
mon in plants of saline habitats. Succulence is particularly general.
Yet these xeromorphic characters have been shown to be relatively
ineffectual in maintaining low transpiration rates in halophytes.
They must then be able to absorb water in spite of the high salt
concentrations, and this is possible because of their own high salt
contents.

Not all species are equally tolerant, and, therefore, they will
often be found in zones adjusted to the concentrations of salts in
the soil and the plant. Flat areas with uniform salt concentration
may support a constant group of species over their entire extent.
The number of species tolerant to salinity is not great and many
of the same genera are found in all parts of the world where similar
conditions occur (e.g., several Chenopodiaceae). Because certain
species in alkali areas are tolerant to definite ranges of salt concen-
tration and, in addition, to particular salts, they may be rather
positive indicators of soil conditions. There are other species that
are not so limited. In some, the concentration of the cell sap ad-

PHYSIOGRAPHIC FACTORS

185

justs itself to changes in the soil and permits growth under a
variety of conditions. Some can tolerate only small amounts of salt
and do better in its absence, while a few others absolutely require

Fig. 86. Margin of a saline flat in the Smoke Creek Desert, Nev. The
shrub at the margin is the relatively salt-tolerant greasewood (Sarcobatus
vermiculatus). Extending farther into the playa is salt grass (Distichlis
stricta), which is more tolerant but soon also fades out until nothing grows
over most of the area— Photo by W. D. Billings.

salt to survive, some even requiring a fairly high concentration.
The extreme in salinity is illustrated by portions of the Great Salt
Lake area in Utah where salt concentrations are so great that no
vascular plants can grow.

TOPOGRAPHY
Although topography affects vegetation indirectly by modify-
ing other factors of the environment, it has nevertheless a signifi-
cant influence upon all plant communities. If an area is so level that
topographic variations are practically nonexistent, then, other fac-
tors being equal, uniform vegetation may be anticipated through-

186 THE STUDY OF PLANT COMMUNITIES • Chapter VII

out. Normally, however, such areas of any extent are rare, and
slopes, bluffs, and ridges with different exposures, lowlands, drain-
age lines, and depressions are present.

Such irregularities in topography produce light, temperature,
and moisture conditions that differ greatly between north and
south slopes or ridges and depressions. The effect of exposure on
these individual factors having been previously discussed (p. 124,
132), it is necessary here only to emphasize that vegetation on
slopes is the resultant of interaction of light, temperature, and
moisture differences. South-facing slopes receive more light, have
higher temperatures, and are drier than the average site in the
area, while north-facing slopes receive less light, are cooler and
moister than the average. Of course, these differences vary with
degree and extent of slope, but, in general, the environment of
north and south slopes differs sufficiently to maintain distinctive
vegetative types.

Apart from the interaction of the factors mentioned above,
slopes affect runoff and the amount of soil water and, likewise, the
possibility of erosion.

Since water always moves toward depressions, they are invari-
ably moister than uplands and usually support distinctive vegeta-
tion. If topography is immature, as in the northeastern United
States, drainage is relatively poor and depressions contain ponds
or lakes supporting aquatic vegetation. Some lakes fill with sedi-
ment, marl, and organic materials to form bogs, which likewise
have their characteristic species. With more mature topography,
depressions are connected by streams, which make drainage far
more effective. Even so, the streams are usually bordered by flood
plains supporting vegetation requiring more favorable moisture
conditions than obtain upon the uplands.

The greatest differences in vegetation associated with local
variations in topography can usually be correlated with moisture,
either in respect to an excess or to a deficiency. If the latter,
adaptations that facilitate absorption or restrict transpiration are
likely to characterize the plants. In a region where moisture is
rarely a critical factor, slope and exposure produce scarcely no-
ticeable differences in vegetation. This occurs only under condi-
tions where a combination of fog, clouds, or rain maintains a hu-

PHYSIOGRAPHIC FACTORS 187

mid atmosphere, low transpiration rates, and a plentiful supply of
water.

In addition to local topographic effects are those of a regional
nature associated with mountains. The increase in precipitation
and decrease in temperature with increasing altitude result in
vegetational zonation. Within these zones, the local effects of to-
pography again become apparent so that zones lie at higher alti-
tudes on a south than on a north slope and the species of a particu-
lar zone will be found extending downward in ravines and upward
on ridges.

A mountain may affect conditions for growth at some distance
from itself. Some mountains are centers over which rain clouds
form and from which they often move to provide moisture for
surrounding lowlands. At the same time, streams starting in moun-
tains and fed by precipitation there, flow down to valleys below.
Other mountains act as barriers when they lie at right angles to the
prevailing winds, for all the moisture may fall upon the mountain
and none be left for the area beyond. This explains the lack of
moisture in the Great Basin. The prevailing winds coming from the
Pacific lose their moisture over the Coast Ranges and the Sierra
Nevada.

Finally, it is probable that mountains act as barriers to the nat-
ural migration of some species that are unable to compete with the
flora upon the mountain or to withstand the successive changes of
environment associated with increasing altitude.

GENERAL REFERENCES

L. D. Baver. Soil Physics.

K.D. GLINKA. The Great Soil Groups of the World and Their Development.

H. JENNY. Factors of Soil Formation.

C. E. KELLOGG. Development and Significance of the Great Soil Groups of

the United States.
R J. KRAMER. Soil Moisture in Relation to Plant Growth.
C. F. MARBUT. Soils of the United States, in Atlas of American Agriculture.
U. S. Dept. Agr. Soils and Men.

CHAPTER VIII

BIOLOGICAL FACTORS

Associated organisms having mutual relationships to each other
and to their environment are recognized as a community. Many,
if not all, of the organisms in a community are thus not only a part
of the community but also a part of the environment of every
other organism there. The dominants obviously compete with
each other and with subordinate individuals. At the same time,
they provide conditions that permit the survival of lesser organ-
isms, which, though quite inconspicuous, may yet markedly affect
the permanence of the community as a whole. Both plants and
animals are factors of the environment of any community, and
man is not the least of these factors.

PLANTS AS FACTORS

Competition.— It has been shown that, within a community,
competition occurs between individuals of the same species, or
between different species, whenever some requirement of the
organisms is available in amounts insufficient to supply all demands
adequately. Each organism involved in competition is a factor in
the environment of all other organisms so involved. The effects of
competing organisms upon each other are more apt to result from
their influence upon physical or physiological conditions of the
environment (such as available water or nutrients, light, tempera-
ture, humidity, and air movement) than they are from direct ac-
tion. An extreme example of direct competition as a factor is that
of the strangling fig, a liana of tropical forests, which climbs to the
tops of the dominant trees that support it. Eventually the tree is
killed as the pressure of the vine about its trunk increases. When
the tree falls, the vine may pull down numerous other trees over
whose tops it has sprawled. The community, however, is only
locally disturbed and soon readjusts itself, for the forest is climax
and these giant lianas are a part of it.

The introduction of new species into a community, by man or

188

BIOLOGICAL FACTORS

189

other agents, usually results in failure because the plant cannot
meet the competition of the normal species, which are adapted to
each other and their environment. However, an occasional species
reverses the rule, establishes itself as a part of the community, and
often produces community changes. Japanese honeysuckle was
introduced in the southeastern states manv years ago and has

FlG. 87. Japanese honeysuckle in bottomland hardwood forest. When the
vine is as dense as this, few tree seedlings come up through it. If they do,
they are soon pulled over and the honeysuckle forms mounds upon them, as
at the left.— Photo by L. E. Anderson.

spread widely. In lowland woods particularly, it sprawls over all
the low vegetation and climbs well up into the trees. Under favor-
able conditions, it almost excludes low herbs and shrubs. When a
tree seedling grows through it, the vine climbs upon it and bends
down the slender stem, which, under the mass of honeysuckle,
soon dies. Such lowland stands frequently have practically no tree
reproduction beneath them. It is a matter of ecological interest as
to how the natural development of these stands will progress. An
economic aspect must be considered by the forester who is inter-
ested in regeneration of trees or planting these areas after cutting,

190 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII

FlG. 88. Dead chestnut, killed by blight, in a forest stand of which they
once were important members. Cherokee National Forest, Tenn.— U. S. For-
est Service.

for, unless the land is cultivated, the honeysuckle cannot be elim-
inated without considerable trouble.

Parasites.— A parasite is completely dependent upon its host for
its existence and thereby becomes a factor in the environment of a
community. When conditions are favorable for the host, a certain
amount of parasitism can be tolerated with little apparent effect.
Parisitic fungi and bacteria are almost constantly present but cause
no serious disturbance of a community unless conditions become
unusually favorable for their increase. Then they may cause death
of enough hosts to produce a change in dominance or to destroy
the community. Such occurrences are usually local and may be
followed by gradual recovery of the original community. How-
ever, when a parasite is introduced from afar, it may be so effec-
tive in its new environment that disaster results.166 Chestnut blight0
has practically eliminated chestnut in the eastern United States,
and oak is now dominant where oak-chestnut occurred before.
Dutch elm disease62 is gradually spreading from New England,
where it first appeared, although its spread has been somewhat re-
tarded by the drastic procedures used to check it.

Parasitic seed plants are not usually of much ecological signifi-

BIOLOGICAL FACTORS

191

cance, but they are always of interest because of their peculiarities
and relatively local distribution. A considerable range of degree of
parasitism is possible.78 The common dodder (Cuscnta) is repre-
sentative of those parasites (holopar ashes) completely dependent
upon their hosts, but the mistletoes and others are termed partial
parasites because they are green and can manufacture food. Some
species are attached to their hosts at a single point of contact, often
by roots. A number of Scrophulariaceae are of this type. Others
twine or sprawl over the host plant and are connected to it at in-
tervals by absorbing structures called haustoria, whose conducting
systems may be in intimate contact with xylem and phloem of the
host. Still others may be contained within the host and show only
their reproductive structures externally. Effects upon the host are
obviously physiological, and reduction of growth and vitality are
usually apparent. Abnormal growth is also common in the pres-
ence of a parasite. It is often manifested as bushy masses, called
"witches brooms" or is occasionally found in twisted, flattened,
or distorted branches. Parasitic seed plants have little effect upon

FIG. 89. A stand of scrubby oak infested with mistletoe (?horadendron
ftavescens).—U S. Forest Service.

192 THE STUDY OF PLANT COMMUNITIES * Chapter VIII

FIG. 90. A striking witches'-broom on a young red pine in Michigan.
U. S. Forest Service.

BIOLOGICAL FACTORS 193

community structure in comparison with the drastic changes that
may result from infestation with pathogenic fungi or bacteria.

Epiphytes.— These include a wide variety of plants, all of which
depend upon larger plants for physical support only. Algae, fungi,
mosses, liverworts and lichens may be found growing on bark or,
in some instances, even on leaves. Often their occurrence seems
correlated only with the general humidity of the atmosphere in
particular habitats, but they are frequently associated with certain
communities and not with others, and, within a community, they
may be distributed systematically. Some may grow only on the
bark of certain trees and, even more specifically, only in patterns
related to drainage of water down that bark.200 Others may be
found only at the base, middle, or top of a tree trunk, and this may
be correlated with moisture content of the bark.23 The occurrence
of the moss Tortula pagorimi8 illustrates how specific a habitat
may be required by some epiphytes. This moss has been found
only in close proximity to man's habitations and then almost exclu-
sively on the trunks of elm trees. The epiphytic lichens associated
with evergreen forests of boreal and alpine regions are distinctive
and characteristic.

In and near the tropics, higher and less variable humidity per-
mits a greater variety of epiphytes to survive, and vascular species
increase. In temperate regions, drought-resistant species, such as
polypody ferns, are found occasionally, but farther south, first on
swamp trees only and then almost anywhere, epiphytic vascular
plants become the rule. Orchids, bromeliads, and ferns are espe-
cially abundant. Structures that catch or conserve water are char-
acteristic of many of these species. Stratification at different levels
in the forest, as controlled by light, air movement, and water sup-
ply, is common, and succession of epiphytic communities may be
observed as organic "soil" is accumulated.181 Occasionally their
weight may increase sufficiently to break down the branches sup-
porting them. Such massive growths as are produced by the well-
known Spanish "moss" (Tillandsia) of the southeastern United
States must reduce the normal foliage and its functioning (see Fig.
8). In general, however, the epiphytes and their "hosts" seem sur-
prisingly well adapted to their relationship.

Symbiosis.— The most generally accepted concept of symbiosis

194 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII

includes only the relationship of intimately associated, dissimilar
organisms that live together to their mutual advantage. By append-
ing descriptive adjectives, the concept has been expanded by some
to include almost any relationship between organisms whether ac-
tually in contact or merely in competition with each other (e.g.,
cattle grazing in a meadow would illustrate antagonistic nutritive
disjunctive symbioses167). But the conservative interpretation rec-
ognizes only a few plant symbionts as significant in community
life. The intimate association of unicellular blue-green algae with
a fungus mycelium, termed a lichen, is an example of plant sym-
bionts that is familiar to all who have any botanical interest.
Lichens, however, can hardly be considered of general importance
in community relationships. Although they often play a part in
the establishment of communities on bare rock, they probably in-
fluence mature, stable communities very little. Fungi and bacteria
living symbiotically on plant roots are less noticeable but of far
more importance.

Mycorhiza — When a root and the mycelium of a fungus grow
together, the fungus may form a feltlike layer around the root

FlG. 91. Transverse sections of mvcorhizal roots of forest trees: (1) en-
dotrophic, (3) ecto-endotrophic, others all ectotrophic. (1 and 4) Psendo-
tsuga imicronata, (2 and 3) Pinus vmrrayana, (5) Popuhis tremuloides, (6)
Picea rubens.— After McDougall and Jacobs.

1G8

BIOLOGICAL FACTORS 195

and penetrate the spaces between cells (ectotrophic mycorhiza),
or the fungus may occur within the cortical cells of the root only
(endotrophic mycorhiza). Such root-fungus relationships are far
more common than was once supposed. It is known that they
occur on most forest trees and shrubs and that many herbaceous
plants may have them. They form during periods favorable to
root growth and are practically restricted to the young roots in
the surface strata of the soil.

Whether mycorhizas represent a mutualistic relationship or
merely parasitism on the part of the fungus has been strongly
argued by numerous investigators. The conflicting evidence makes
interesting, if somewhat confusing, reading. However, the evi-
dence that mycorhiza must be present for the successful growth
of many species, particularly forest species, is sufficient to suggest
that the mycorhizal condition is desirable under most situations
even though the reasons are not too obvious.

Pot cultures of certain tree seedlings in poor soil have been un-
satisfactory until inoculated with mycorhizal fungi. On a larger
scale, unsuccessful forest nurseries on prairie soil or long deforested
agricultural soil have been saved by bringing in small amounts of
forest soil, which started the formation of mycorhiza. Tree seed-
lings transplanted without mycorhiza to treeless areas have been
saved from gradual death by the application of small amounts of
soil containing mycorhizal fungi.

Several members of the heath family (azalea, rhododendron,
blueberry) are dependent upon the presence of mycorhiza that
cannot tolerate alkaline conditions. Disappearance of mycorhiza
leads to death of the plants, and consequently, the soil must be
acid for successful propagation of these species.

Many orchid seeds germinate normally only in the presence of
mycorhizal fungi and were difficult to propagate until it was
found that proper nutrient media could compensate for the ab-
sence of the fungus. Such evidence indicates that, regardless of
what the fungus may take from the root, the vascular plant is
benefited by the presence of the mycorhiza or may actually be
dependent upon it. Probably the benefit is derived through some
nutritional improvement provided by activities of the fungus.

Nodules— Certain saprophytic bacteria, living free in many soils,

196 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII

FlG. 92. Two seedlings of Psychotria punctata, about three and one-half
months old. The plant on the right is normal both as to growth and the pres-
ence of bacterial nodules dotting every leaf. The one on the left, grown bac-
teria-free, has reached its maximum development.— From Hwmn.

127

enter the root hairs of most legumes when available and produce
a proliferation of cortical cells sufficient to appear as a small
nodule on the root. Although the plant provides food for the bac-
teria and produces the nodule in which the bacteria multiply, the
relationship is truly symbiotic. These nitrogen-fixing bacteria are
able to take free nitrogen from the air, unavailable to most
plants, and to combine it with other elements to form compounds
that can be used by the plant during its lifetime. After death of the
plant, the accumulated nitrogenous compounds are released in the
soil and are used by other plants growing there. Legumes and
nitrogen-fixing bacteria are, therefore, important factors in main-
taining soil fertility in natural or cultivated soils. Plant commu-
nities becoming established on poor sites, such as eroded slopes,
invariably include a number of legumes, which are, of course,
particularly adapted to colonizing sterile or nitrate-depleted soils
and contributing to their improvement. Agricultural practice in-
cludes legumes in most crop rotations, and worn-out lands are
rebuilt by cropping with legumes of some sort.

Nodules produced by bacteria are found on the roots of a few
plants in families other than Leguminosae, but they are not of the
same type. Nodules containing bacteria are also formed on leaves
of a number of tropical plants, mostly in the family Rubiaceae.

BIOLOGICAL FACTORS 197

These bacteria are associated with the plant tissues in all stages of
development from seed to maturity, but nodules form only on
leaves. Although these bacteria have been credited with nitrogen-
fixing ability, it is certain that the plants are not dependent upon
them for their nitrates. Certain products of their presence are
necessary, however, for without the bacteria, seedlings do not ma-
ture.127 The relationship is, therefore, truly symbiotic (Fig. 92).

Other Soil Flora.— In addition to the symbiotic fungi and bac-
teria, great numbers of bacteria, fungi, and algae occur free in the
soil. Their importance to natural plant communities cannot be
evaluated accurately, but their significance is indicated by their
general functions of making nitrogen available by fixing it, or
releasing it with other nutrients through their activities in decom-
posing organic matter.

The fixation of nitrogen as nitrates by free soil organisms is
known to be accomplished by a number of bacteria under both
aerobic and anaerobic conditions and even in practically sterile
soils. Some are inhibited by acidity or chemical constituents of the
soil, and temperature ranges may affect their activity, but, in gen-
eral, some are present almost everywhere. Certain algae are also
thought to be capable of nitrogen fixation.

All nitrates appearing in the soil from sources other than fixa-
tion are the products of organic decomposition, particularly of
proteins. The breakdown involves a series of chemical changes
accomplished by a succession of bacteria and fungi. The first of
these causes the proteins to break down into the less complex pro-
teoses, peptones, and amino acids. This digestive process allows
the bacteria and fungi to use a part of the nitrogen for themselves,
and, in so doing, they release ammonia as a waste. Few plants can
use ammonia directly, and many are injured by its accumulation
in the soil. Ammonification is followed by nitrification, in which a
group of nitrite bacteria convert the ammonia to nitrites by partial
oxidation. Subsequently, the activities of nitrate bacteria cause
further oxidation and the formation of nitrates. Now, finally, the
nitrogen is usable by higher plants. Digestion of proteins, am-
monification, and nitrification must all take place before organic
nitrogen can be used by plants, and the succession of bacteria must
be present if the processes are to occur.

198 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII

The activities resulting in available nitrates produce partial
breakdown of organic materials, which are further decomposed
by other bacteria and fungi acting upon the remaining nonprotein
plant materials. The partially decomposed plant remains, or hu-
mus, may be broken down completely in a single season if rela-
tively high temperatures and sufficient moisture occur most of the
year and permit more or less continuous functioning of the organ-
isms. If the organisms can operate for only a few summer months,
the deposition of litter usually exceeds the rate of decomposition,
and humus tends to accumulate.

ANIMALS AS FACTORS

Pollination.— Insects are by far the most important animals in-
volved in pollination, and bees, wasps, moths, and butterflies are
particularly concerned. A few birds, especially hummingbirds,
contribute to pollen transport, and even some small crawling ani-
mals may be effective at times. Most animal-pollinated flowers
have certain characteristics in common, such as conspicuousness
in size and color and the production of an odor as well as nectar.
It has been shown that all of these characters serve more or less to
attract insects. In general, the flowers are more elaborate than
those of wind-pollinated plants, and they have characters usually
interpreted as of more modern origin.

Devices that insure insect pollination are common and often of
intricate design. Adaptations may occur in both insect and flower
limiting pollination of a particular species to a single type of insect.
Some adaptations are so extreme as to produce complete depen-
dence of plant and insect upon each other.

Dissemination.— Plant parts, called disseminules, give rise to new
individuals in new places. Their food content is an attraction to
various animals, which, consequently, often act as agents of dis-
semination. Many seeds that are eaten are indigestible and retain
their viabilitv after they are dropped at considerable distances
from their sources. Others, not immediately eaten, are carried by
birds, rodents and even ants to places of storage or concealment,
where they may germinate. Of course, great numbers of seeds are
eaten or destroyed by animals, but dissemination from seed sources
is a partially compensating factor.

BIOLOGICAL FACTORS

199

Vegetative structures may be effective in the same way. Aqua-
tic animals, such as muskrat, tear up rhizomes and bulbs, some of
which float free and establish new communities elsewhere. In this
connection, it is worth mentioning the importance of water as an
agent of dissemination, especially of floating propagules, even
though they do not retain their viability for long when saturated.

Finally, the hooks, spines, and other devices characteristic of
many seeds and fruits insure their attachment to almost any ani-
mal contacting them and thus make possible their transport for

FlG. 93. Structural modifications of seeds and fruits that facilitate dissem-
ination by wind or animals. (1) The parachute fruit of common dandelion
(Taraxacum) ; (2) winged fruit of dock (Rumex pulcher); (3) the silky-
haired seed of milkweed (Asclepias mexicana); spiny, hooked, and awned
fruits of (4) sandbur (Cenchrus paucifloriis) , (5) cocklebur (Xanthium
canadense), (6) red-stem filaree (Erodium cicutarium), (7) beggar's-tick
(Widens frondosa).— By permission, from Weed Control by Robbins, Crafts,
and Raynor, copyrighted 1942, McGraiv-Hill Book Company.

200 THE STUDY OF PLANT COMMUNITIES ' Chapter VIII

some distance. Animals with long, soft hair are the most effective
agents. The clothing of man is likewise well adapted to such
transport, as anyone knows who spends time in the field during
late summer and fall. Some of these devices are simple hooks, ef-
fective because of sharpness or strength; others are elaborate
structures with several features insuring their transport. The fruits
of awn and needle grasses are illustrative, since they have sharp-
pointed, retrorsely-barbed fruits, which easily penetrate cloth,
fur, or wool, and an awn which twists with changes of moisture
and thus pushes the fruit forward to a secure anchorage. These
may cause severe damage to grazing animals by penetrating skin,
lips, or even internal organs.

Soil Animals.— The microfauna of the soil, concentrated in the
upper strata, consists of great numbers of protozoa, nematodes,
and rotifers. In addition, there are various macroscopic worms and
insects.263 In general, the numbers of animals vary in response to
the same factors affecting the microflora, and the greatest numbers
are always found in soil with high organic content. All contribute
to organic decomposition and use a part of the products for food.
Several protozoa probably consume bacteria, and some nematodes
are parasitic on the roots of plants, causing much trouble in culti-
vated soils where they are present.

Of the macroscopic fauna, earthworms are most active. Their
constant burrowing facilitates aeration and drainage and their use
of fresh or partially decomposed organic matter as food contrib-
utes to decomposition. Since mineral matter is also ingested in
feeding, the earthworm moves quantities of soil about, and this
tends to mix mineral and organic materials. In cultivated soils this
has no great significance, but for natural soils the advantages are
obvious. Earthworms are found in the best soils and best sites but
rarely in poor soils. It would appear, then, that they serve to make
good soils better but that poor soils derive little from them.

A very high proportion of all insects spend part of their lives in
the soil. Their larvae tunnel through the soil and, thereby, con-
tribute to organic decomposition and distribution.

Larger Animals.— The principal effect of larger animals upon
plants results from grazing or other feeding habits. Carnivorous
animals affect communities onlv indirectly by keeping down the

BIOLOGICAL FACTORS

201

population of herbivores and thus maintaining a balance in food
relationships. In spite of this, the feeding by herbivores may some-
times be excessive enough to cause serious disturbance or even
destruction of community structure.

Under natural conditions, grazing was undoubtedly greatest
when buffalo ranged throughout our grasslands. Locally, as around
water holes, their feeding and trampling sometimes destroyed

FlG. 94. Distinct browse line on stand of ironwood resulting from deer
feeding on low branches. Note the uninterrupted view under stand, and ab-
sence of shrubs and tree seedlings. Such damage commonly results when deer
population is high, and especially when winter supply of food is inadequate.
— U. S. Forest Service.

most of the vegetation but otherwise probably did little damage
since they were constantly on the move and distributed themselves
where grazing was best. Moderate grazing by cattle does not
change the essential nature of a grassland community. A succession
of dry years in the time of the buffalo could have resulted in local
conditions similar to those in overgrazed pasture areas today.

Deer and moose similarly have little effect on grassland or for-
est, where they browse, unless there is an overpopulation. Then,
especially as a result of winter browsing, the complete destruction
of tree reproduction might be possible.

Prairie dogs may consume all the forage for some distance about
their villages. The total consumption of food bv such relatively
small animals is sufficient to reduce considerably the value of a
range for larger herbivores. The same may be said for jack rabbits,
but their feeding is less localized.

202 THE STUDY OF PLANT COMMUNITIES • Chapter VIII

The feeding of cottontail rabbits ordinarily affects natural vege-
tation but little. However, if a peak in their fluctuating population
comes at the time of a bad winter with much snow, they can do
serious damage to seedlings and even to larger trees from which
they eat the bark. Because of selective feeding, snowshoe rabbits
may change the course of forest succession.68

FlG. 95. Injuries to seedlings and saplings resulting from feeding by ro-
dents and larger animals may strongly influence the development of stands
and the nature of future vegetation. (1) Young ponderosa pine girdled by
porcupine. (2) Scotch pine browsed by deer the year after planting. All
needles and buds eaten. (3) A pine seedling eaten back by rabbits in three
successive winters. Such seedlings can never make normal trees.— U. S. Forest
Service.

Rodents that eat bark by preference may cause considerable
damage, especially if their feeding is selective as to species. Porcu-
pines are in this category, and beavers are even more destructive
because their activities are concentrated around their dams. Here
they cut down and strip the bark from the trees they most prefer
nearest their ponds and then gradually extend their operations to
surrounding slopes. Their dams, too, affect conditions locally, for
they maintain ponds that sometimes flood large areas, modify
drainage, and even affect the water table. This may sometimes be
desirable, sometimes not.

Man.— The effects of man upon vegetation are fundamentally
similar to those of lower animals. The greater the concentration of

BIOLOGICAL FACTORS

203

population, the greater the modification of natural communities
by use and destruction. Whereas man was once essentially a de-
pendent in community structure, he is now more and more be-
coming the dominant organism everywhere. By cultivation, he has
eliminated natural vegetation from vast areas. Logging, even with-

FlG. 96. Center of a burned swamp in iMaryland that once supported ma-
ture cypress-gum forest. Intense fire destroyed the forest and burned deep
into the peaty soil, which had accumulated through the years. Rebuilding
soil in the depressions, now filled with water, will require many years and
numerous generations of plants.— Photo by G. E Beaven.

out subsequent cultivation, has changed the forests, and stands
equaling the original virgin forests will probably never occupy
most logged areas again. Cities, highways, airfields, and similar
products of man's living mean serious disturbance of natural vege-
tation. Drainage and irrigation projects, canals, road fills, and dams
result in soil moisture changes that promote the development of
quite different communities. Many similar disturbances can be
noted as a result of animal activities but always on a more local-
ized scale and consequently with less permanent effects.

Fire is not peculiar to man's activities and, undoubtedly, oc-
curred here and there in North America before the white man
came. However, the conditions provided by lumbering operations,
and the constant use of fire, often with too little concern for its

204 THE STUDY OF PLANT COMMUNITIES • Chapter VIII

effects, have made it an important factor associated with man's
presence. Local small fires occur almost everywhere occasionally,
and the destruction of vegetation followed by gradual replace-
ment is characteristic. Under the right conditions, fire may be so
common as to become a major factor controlling the vegetation of
a region. This is true of much of the coastal plain of the south-

FlG. 97. What fire can do to a mountain forest. Such fires are usually fol-
lowed by erosion, and it requires years for the re-establishment of forest
vegetation. Coconino National Forest. Ariz.— U. S. Forest Service.

eastern United States.105 Prolonged dry periods and little attempt
to control fire in these flatlands result in most areas burning almost
every year. Only fire-resistant species predominate and only a
limited degree of vegetational development is possible before fire
occurs again and sets back that development. As a result, grassy
savannahs with longleaf pine are characteristic instead of the po-
tentially possible hardwood forests. In parts of California, fires
have resulted in an increase of the fire-resistant chaparral and a
proportionate decrease of forest. Similar illustrations may be
found in many parts of the world.
The immediate economic loss from an intense forest fire is

BIOLOGICAL FACTORS

205

paralleled by other less obvious losses. Such fires in the temperate
zones may destroy practically all the humus accumulated through
the years and necessitate the slow rebuilding of the soil before
forest can occupy the area again. Leaching and erosion, which
follow such fires, may delay revegetation for years. Thus the pro-
ductivity of the soil may be indefinitely impaired.

FlG. 98. A subalpine flat denuded by intense fire that killed all trees and
burned off organic material down to mineral soil. The fire occurred many
years before picture was taken and it is obvious that it will be many more
years before the soil is sufficiently rebuilt to support forest.— U. S. Forest
Service.

It is of interest that light, controlled burning has been found
beneficial for certain purposes. On some grazing land, certain
undesirable species may be kept down or eliminated to the ad-
vantage of more palatable plants. More vigorous growth of certain
forage types is sometimes obtained after light burning in the
proper season, probably because of the nutrients released and made
available. It would appear that under some circumstances fire
could be used as a beneficial tool.117

Man, like lower animals, transports seeds and fruits, but to far
greater distances and with resulting changes in vegetation of a

206 THE STUDY OF PLANT COMMUNITIES • Chapter VIII

^5 *

FlG. 99. An introduced weed, tumble mustard (Sisymbrium altissmtum),
dominant over the entire extent of a sagebrush burn, one year after the fire.
Washoe County, Nev.— Photo by W. D. Billings.

more drastic nature. It is hard to believe that 60 percent or more
of our weeds are not native but introduced species that have come
from all parts of the world.176 Some were brought in as orna-
mentals and almost immediately escaped and spread from gardens.
Others came in accidentally with seeds of desirable plants. Many
introductions have been useful and extremely valuable. Most of
our cultivated plants have been improved by crossing with strains
of foreign varieties at some time, or they were themselves original-
ly introduced. In recent years, such introductions are not made
haphazardly.

Unfortunate experiences with unconsidered or accidental intro-
ductions can be listed for all parts of the world. The water hy-
acinth, introduced from South America, has spread throughout
the lowland waterways of our southern states where it chokes
canals, impedes drainage and navigation, and destroys wildlife. A
similar problem has resulted with the introduction of Elodea in
the low countries of Europe. Animals may cause similar difficul-
ties, as the spread of the introduced English sparrow and the star-
ling in the United States. The muskrat has become a pest in central

BIOLOGICAL FACTORS

207

Europe, and rabbits, introduced into Australia, increased to
enormous numbers in only a few years.

Natural communities are made up of groups of species adapted
to living together. The numbers and sizes of individuals are de-
termined by the entire complex of environmental factors. If a
species is eliminated, others of the community may increase and
take its place, or there may then be opportunity for an incidental
species to become a part of the community. Usually, if a species is
introduced, it does not reproduce and gradually dies out. Occa-
sionally, an introduced species has the necessary characteristics to
compete successfully and to reproduce regularly. Then adjust-
ments must be made within the community and a new balance
among its members must be established. Such a species might even
become a dominant, and then the adjustments would result in a
new community. The prickly pear (Opuntia inermis), introduced
in Australia, became a dominant and made useless more than thirty
million acres in Queensland alone.

FIG. 100. Massed water hyacinth covering the water in Louisiana swamp-
land. The dusting by airplane is part of an experimental eradication program.
-Courtesy of Department of Wildlife and Fisheries, Louisiana.

208 THE STUDY OF PLANT COMMUNITIES ' Chapter VIII

When man has tampered with the balance among the species of
a community by eliminations or introductions, he has not always
considered the possible effects upon the community as a whole. If
large carnivores are destroyed, herbivores increase, and, if their
reproductive capacity is great, they may soon become so abundant
that their grazing destroys the community or changes it radically.
If a predator is introduced whose prey is some native species that
is a pest, the predators may eliminate the pest and then become
pests themselves.10

Only a few examples are necessary to illustrate these points. The
Indian mongoose was introduced into Haiti, Jamaica, and other
West Indian islands to rid them of rats and snakes. This the mon-
goose did most effectively, but its numbers increased, and, with its
natural prey disappearing, it turned to robbing birds' nests of eggs
and young. Now it is practically impossible to raise poultry there.
The gypsy moth was accidentally introduced into Massachusetts
when it escaped from cultures being reared to test its silk-produc-
ing ability. It is now a serious pest of fruit and shade trees in most
of the eastern United States although much money and effort have
been expended to control it. On the other hand, introductions of
about sixty foreign predators or parasites of the gypsy moth have
resulted in the establishment of a dozen or more that are aiding in
its partial control. The destruction of coyotes in some western
states has resulted in such marked increase of rabbits that their
winter feeding on tree seedlings modifies vegetational develop-
ment (see Fig. 95).

On game reserves where predators have been eliminated and no
hunting is permitted, the population of herbivores, such as deer,
usually increases rapidly. When the number of deer exceeds the
natural carrying capacity of the region, a shortage of food results
during unfavorable seasons. Then, especially in winter, many ani-
mals die unless they are fed bv man. As a result of supplementary
feeding, the population is still larger the next season, and the prob-
lem is not solved. Controlled hunting is now permitted on several
such reserves where the population capacity has been determined.
The effects on the vegetation of such overcrowding are very con-
spicuous. All young woody plants protruding above snow are
eaten off, and the lower limbs of young trees, even conifers, are

BIOLOGICAL FACTORS

209

"pruned" to the height the animals can reach, standing on their
hind legs. Obviously, community structure and development in
such areas is completely out of balance.

Disturbance of natural communities should not be undertaken
without a reasonable appreciation of the end results. Management
or manipulation of the balance among species of a community may

FlG. 101. Drained swampland in the Everglades of Florida. Many acres of
these muck soils are producing winter truck crops in quantities, now that
problems of drainage, tillage, and fertilizing have been worked out.— U. S.
Soil Conservation Service.

frequently be possible but should offer the best prospects of suc-
cess when the ecology of the individuals and the community is
well understood.

Man's unconcern for natural resources built up through the
years has led to economic losses and a reduction of those resources,
which only time can replace. Soil erosion, quite unnecessary if
cropping is properly handled, had reached a shameful point before
we began to do anything about it. Only recently have we at-
tempted to correct overworking of poor soils, mismanagement of

210 THE STUDY OF PLANT COMMUNITIES * Chapter VIII

others, overgrazing, and other destructive practices. Contour
plowing, strip cropping, terracing, and similar procedures check
runoff, hold water, and permit the rebuilding of rundown soils.
On wild lands and some submarginal cultivated lands, the re-estab-
lishment of natural vegetation is being encouraged where it should
never have been removed. Application of ecological principles in
such reclamation has generally paid good dividends.

Not only has man disturbed or destroyed natural vegetation,
but he has also modified the environment, sometimes to his ad-
vantage. By irrigation or drainage, the soil moisture has been so
modified that great acreages have been brought under his control.
Enormous dams hold water in artificial lakes. When this water is
properly supplied to the surrounding soils, it transforms worthless
desert to highly productive agricultural land. Elsewhere drainage
systems put into lowlands have changed swampy, untillable soil to
some of the best truck and farming acreages. Not all drainage
projects have been profitable, however, especially those of muck
lands. Not all are equally productive, and cost of maintaining
drainage of some mucks is out of proportion to the crop yields.
Many such projects have been abandoned— to the joy of sports-
men and conservationists, who objected to the extensive destruc-
tion of homes and feeding grounds of all kinds of wildlife associ-
ated with these swamps.

GENERAL. REFERENCES

R. M. ANDERSON. Effect of the Introduction of Exotic Animal Forms.

J. M. Coulter, C. R. Barnes and H. C. Cowles. A Textbook of Botany.

(Vol. II : Ecology, pp.485-964.)
H. C. HANSON. Fire in Land Use and Management.
XV. A. McCUBBIN. Preventing Plant Disease Introduction.
S. A. WAKSMAN. Principles of Soil Microbiology.

Part 4 - Community Dynamics

CHAPTER IX

PLANT SUCCESSION

HISTORICAL BACKGROUND

When a cultivated field is permitted to lie fallow, it produces
a crop of annual weeds the first year, numerous perennials the sec-
ond year, and a community of perennials thereafter. In forest
areas, the perennial herbs are soon superseded by woody plants,
which become dominant. After any disturbance of natural vege-
tation—such as cultivation, lumbering, or fire— a similar sequence
of communities occurs with several changes in the dominant vege-
tation through the years.

Such relatively rapid vegetational changes are familiar to most
people today and must have been observed hundreds of years ago.
It was not until the seventeenth century, however, that any syste-
matic study of such changes was made, and those studies dealt
primarily with the development of peat bogs. Bog studies were
continued in the eighteenth century, and, in addition, some at-
tempt was made to apply the principles to burned and disturbed
upland areas. It was then that the term, succession, was first ap-
plied to the vegetational changes involved. During the nineteenth
century, succession was considered rather frequently but invari-
ably as incidental to other problems. Several writers hinted at the
importance of succession in all habitats, but it was not until 1885
that a regional study of vegetation in Finland was made in which
succession was recognized as fundamental to all community de-
velopment.

Between 1890 and 1905, the modern concepts of succession
were clarified through the efforts of several writers. Two, whose
influence has been as great as any, were Americans. In the first
comprehensive application of successional principles in the United

211

212 THE STUDY OF PLANT COMMUNITIES * Chapter IX

States, Dr. Henry C. Cowles (1899) described the development of
vegetation on the sand dunes of Lake Michigan. Later (1901) he
described the vegetation of Chicago and vicinity, as it is related to
physiography, in so logical a fashion that a pattern for studies of
community dynamics was established. His papers also served to
stimulate similar investigations by others. Beginning at about the
same time, the publications of Dr. F. E. Clements, then working in
Nebraska, included much that served to shape our present con-
cepts of succession. The culmination of his ideas appeared in his
exhaustive treatment of the entire subject of plant succession,56
which remains a basic source of reference today.

THE CONCEPT

Plant communities are never completely stable. They are char-
acterized by constant change,73 sometimes radical and abrupt,
sometimes so slow as to be scarcely discernible over a period of
years. These changes are not haphazard, for within a climatic
area, they are predictable for a given community in a particular
habitat. This means, of course, that similar habitats within a cli-
matic area support a sequence of dominants that tend to succeed
each other in the same order. Contrasting habitats do not support
the same sequence of communities. As a result, any region with
several types of habitats will have an equal number of possible suc-
cessional trends.

CAUSES

A detailed consideration of the relationships of organisms to
their environment should make it clear that major changes in the
composition of a community can only follow changes in the en-
vironment. The specific, immediate cause of a particular change
of species may not always be obvious because of the interrelation-
ship of controlling factors. Two general types of habitat change
may cause differences in the community. Development of the
community causes parallel developmental changes of the environ-
ment, and physiographic changes can likewise modify the envi-
ronment materially.

Developmental changes of the environment result from reac-
tions upon the habitat by the organisms living there. To illustrate :
Accumulation of litter affects runoff, soil temperature, and the

PLANT SUCCESSION

213

formation of humus; this, in turn, contributes to soil development,
modifies water relations, available nutrients, pH, and aeration,
and affects soil organisms. Thus every organism in a community
may have some reaction upon the habitat. By these reactions, the
habitat becomes changed and consequently is less favorable to the
organisms responsible for the changes, while, at the same time, it

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PRIMARY SUCCESSIONS

FlG. 102. A diagram of the trends of succession for the principal habitats
on Isle Royale, Lake Superior. This is one of the early complete condensa-
tions of a successional story for an entire region. On this pattern, similar dia-
grams have been worked out for many sections of the country. Note that the
system shows at a glance the kinds of habitats in which succession originates,
the interrelationship of trends, and the major dominants in each of the stages
of succession. Study of the diagram should help to clarify concepts of suc-
cession and climax. It must be remembered that not all trends progress with
equal speed.— After Cooper.10

has become more favorable for species that could exist there
previously only with difficulty. Under the changed conditions,
new species are able to compete successfully with the established
species and often even to replace them.

The habitat may also be modified by forces quite apart from
the effects of organisms. A flood plain or swamp may become bet-
ter drained as a stream cuts more deeply into its channel. Silting in
of a lake or pond raises the level of mineral soil. Chemical changes
in the soil may result from leaching or accumulation of salts. Such
modifications of the habitat also produce vegetational changes.

214 THE STUDY OF PLANT COMMUNITIES ■ Chapter IX

These two types of causes of succession are commonly in opera-
tion at the same time, and their effects cannot always be readily
separated. Since they both result in vegetational change, it seems
unnecessary to distinguish between their effects in a general con-
sideration of plant succession.

KINDS OF SUCCESSION

Primary succession is initiated on a bare area where no vegeta-
tion has grown before. It may be observed on glacial moraine ex-
posed by recession of the ice, a new island, an area of extreme
erosion, newly deposited volcanic ash or rock, or any similar habi-
tat newly exposed to colonization. Such habitats are apt to be
unsuitable to the growth of most plants, and, consequently, the
pioneers that do establish themselves must have adaptations per-
mitting survival under extreme conditions. Moisture relationships
usually control their ability to invade the new area. If the habitat
is extremely dry, it is described as xeric; if wet, hydric; and if
intermediate, mesic. The successional trends are similarly referred
to as being xerarch, hydrarch or mesarch succession.

Whatever the condition of the initial habitat, reaction of vege-
tation tends to make it more favorable to plants and always results
in improved moisture conditions. Thus xeric habitats become
moister and hydric ones become drier as succession progresses.
Because of the diversity of habitats upon which succession may
begin, there are an almost equal number of possible pioneer com-
munities. Within a climatic area, however, the variety of commu-
nities decreases as succession progresses because the trend is to-
ward mesophytism from both hydric and xeric habitats. Thus
unrelated habitats may eventually support similar vegetation and
may even undergo identical late stages of succession.

Secondary succession results when a normal succession is dis-
rupted by fire, cultivation, lumbering, wind throw, or any similar
disturbance that destroys the principal species of an established
community. To what extent the development of vegetation on the
secondary area resembles primary succession is determined by the
degree of disturbance. Although the first communities that de-
velop may not be typical of primary succession, the later stages
again are similar. When disturbance is extreme, as after severe fire,

PLANT SUCCESSION

215

FlG. 103. An illustration of relatively rapid secondary succession. The
fire that destroyed this Oregon forest (above) did not appreciably affect the
soil organic matter and was not followed by erosion. As a result, Douglas fir
soon became established and, when fourteen years old, formed a closed stand
10 to 15 feet tall (below).— U. S. Forest Service.

many of the effects of previous vegetation upon the habitat are
eliminated, resulting in a slow vegetational development. After
wind throw or lumbering, many of the products of community
reaction remain and succession is rapid. If seedlings and young
trees are not destroyed, progress of succession tends to exceed that
of the original trend.

216 THE STUDY OF PLANT COMMUNITIES ■ Chapter IX

Most of the settled parts of North America have little evidence
of primary succession today, and even unsettled areas have largely
been disturbed by grazing or lumbering. Thus primary succession
must often be interpreted in terms of small and often poor exam-

FlG. 104. Hydrarch succession as illustrated by girdles of vegetation
around a shallow lake in northern Minnesota. In what remains of open water
are submerged and floating-leaved aquatics, the pioneer angiosperms. A mar-
ginal, floating sedge mat is gradually filling the lake with peat and advancing
over the water. On the mat are a few bog shrubs, behind which is a girdle of
tamarack forming a closed stand. The oldest part of the bog is marked by
the spires of black spruce, which succeed the tamarack. On the upland, be-
hind the spruce, is a mixed white pine-hardwood forest. Eventually, the en-
tire depression will be a peat-filled bog supporting a forest of black spruce.

pies of what once occurred. Studies of secondary succession may,
however, have the greatest practical value because we are in-
volved with secondary successions in any problem of applied
ecology; yet their interpretation may be partially dependent upon
an understanding of primary successions.

Representative Successions.— Because water and bare rock rep-
resent the extremes in types of habitats upon which succession is
initiated, the growth form of early stages of each is remarkably
similar everywhere and even genera and some species are often

PLANT SUCCESSION

217

duplicated regardless of the region. It is, therefore, possible to pre-
sent a general description of such successions, which can be ap-
plied almost anywhere and which will illustrate what we have just
discussed.

FIG. 105. Hydrarch succession illustrated by swamp vegetation. The zone
of cattails occupies the partially flooded, muddy margins. When soil builds
up or drainage improves, bog shrubs (buttonbush, alder, willow) appear as
in the middle background. On wet, but drained soil a swamp forest of mixed
hardwoods develops as in background.-Ptoo by H. L. Blomquist.

Hydrarch succession progresses in response to better moisture
conditions in combination with improved aeration. Initiated in a
lake, pond, or stream margin where water movement is not too
great, the pioneer vascular plants are submerged aquatics with
thin, dissected, or linear leaves. Their depth of growth is limited,

218 THE STUDY OF PLANT COMMUNITIES • Chapter IX

FlG. 106 (A). Xerarch succession as illustrated by vegetational develop-
ment on granitic rock in the Piedmont of the southeastern states. Early stage
(upper) of mat formation initiated by the pioneer moss (Grimmia laevigata)
upon which a lichen (Cladonia leporina) is well established. As mat thickens
(lower), herbs come in, with eventual Andropogon spp. dominance.1

186

on the one hand, by light penetration of the water and, on the
other, by a zone of floating-leaved species. These latter (water
lilies, etc.) exclude submerged species by shading but cannot move
into the zone of submerged forms until the bottom is built up or
the water level falls. In still shallower water, emergent species pre-
dominate. These have their roots and rhizomes in the mud and
extend upward into the air (rushes, reeds, cattails, sedges). The

PLANT SUCCESSION

219

close growth in this zone serves to hold sediment, and the bulk
results in substantial accumulation of partially decomposed or-
ganic matter. When filling is sufficient, shrubs can survive on the
built-up soil. Finally, the soil will be firm enough and sufficiently
raised above the water table to support lowland trees, which may
eventually give way to a community similar to that of uplands.
This entire sequence can sometimes be seen as a more or less

wm.

FlG. 106 (B). Shrub stage of rock succession, mostly Rhus copallina here.
Note fringe of Andropogon, smaller herbs, and finally mosses at periphery
(upper). Tree stage (lower) on an old mat, forming an island on bare rock.
Oak-hickory forest in background is growing on shallow soil overlying
rocks.186

220 THE STUDY OF PLANT COMMUNITIES ■ Chapter IX

continuous series of zones girdling a lake that is gradually filling
in. Borings of the soil under any zone will show the partially de-
composed remains, in vertical sequence, of each of the previous
stages of succession that contributed to the development of that
zone.

Xerarch succession on rock follows a definite pattern, whose
progress is controlled by the rate at which soil forms and accumu-
lates. Pioneers on rock surfaces are either lichens or mosses ca-
pable of growing during the brief periods when water is available
to them and lying more or less dormant through the usually longer
periods of drought. The pioneer lichens are crustose and foliose
types, which usually contribute little to succession since they are
not mat-forming.186 However, they do probably cause corrosion
of the rock surface and thus provide some anchorage for other
species. Pioneer mosses, on the other hand, are in tufts or clumps,
which catch dust and mineral matter from wind and water. This
material, combined with the remains of mosses, forms a gradually
thickening mat with a periphery of young plants that spreads over
bare rock (and the pioneer lichens) and with a central area that
may become thick enough to support foliose lichens (Cladonia
especially), larger mosses such as Polytrichum, or often species
of Selaginella. Such bushy plants catch and hold still more mineral
material, and their death adds much organic soil to the mat.

When soil has built up sufficiently to provide the necessary an-
chorage and water-retaining ability, seed plants appear on the
mats. A number of hardy, annual herbs, often weeds of field and
garden, appear first and are followed by biennials and perennials,
of which grasses are most abundant. Later a shrub stage becomes
dominant, which usually includes some species of sumac (Rhus)
and several ericaceous shrubs. By this time, the mats may be sev-
eral inches or a foot thick and then trees make their appearance.

Just as a series of girdles of vegetation usually surrounds a lake
and indicates the sequence of succession from open water to solid
ground, so the progress of succession on rock may be seen as a
series of girdles of vegetation from the periphery to the center of
an old mat. Pioneers are at the outer margin of the mat, and each
successive stage of dominance is nearer the center where, on the
thickest soil, trees may be present.

PLANT SUCCESSION

221

FlG. 107. Herb stages in secondary succession on abandoned upland fields
in the Piedmont of the southeast. (1) Horseweed dominance on a field
abandoned one year. (2) Aster dominance indicating two years of abandon-
ment. (3) Broom sedge (Andropogon) dominance in a field abandoned five
years, and young pine well established.

The early stages of these two successional trends are apt to be
extremely slow, but later stages speed up considerably as reaction

222 THE STUDY OF PLANT COMMUNITIES • Chapter IX

FlG. 108. Forest stages of old-field succession (continuing Fig. 107). (1)
Fully stocked 15-year loblolly pine, which has eliminated all old-field herbs
and under which hardwood seedlings may be found, (2) 26-year pine, under
which saplings of gum, red maple, and dogwood are noticeable, (3) 50-year
pine stand in which hardwoods, including oak and hickory, have formed an
understory, (4) oak-hickory climax forest, of the type that could develop on
an old field after 200 years or more.183— Photo (1) by C. E Korstian.

PLANT SUCCESSION 223

of the vegetation becomes more effective. The final changes, after
tree dominance, are again very slow. Changes of currents or drain-
age in the lake and wind throw or fire on the rock may disrupt
either of the trends and result in secondary succession. The result
of succession in both habitats is, however, a gradual change in the
direction of habitat conditions that are relatively mesic for the
climate of the region and a community adapted to such conditions.

RATE OF SUCCESSION

If succession is to be recognized as universal and occurring in all
habitats, it becomes necessary to ignore time to some extent. A
mesic habitat in a given climate will obviously produce a forest
much more quickly than a xeric one, especially if the initial habi-
tat is bare rock. Yet the potential ultimate communities of the two
sites are the same, for all successions in a climatic area progress
toward communities of mesophytes. Two habitats of apparently
similar characteristics might support the same successional se-
quence, but progress of the successions might be at different rates
because of the type of soil and the difference in its response to
reaction. Or, if seed sources were not equally available to both
sites, one might develop more rapidly than another. This could
result from an oversupply of seed, producing overstocking of cer-
tain species and consequent delay in development of the next stage
because of competition; on the other hand, poor seed sources or a
series of poor seed years might materially delay the initiation of a
community that otherwise could have started. This should make it
clear that the rate of succession is extremely variable. Pioneer
stages of primary succession are commonly very slow because
they can progress only with soil development. An extreme exam-
ple is probably that of succession on bare rock, which must wait
not only upon soil development but also upon the disintegration
of the rock for soil formation. In contrast, the pioneer stages of sec-
ondary succession, especially on abandoned fields, are remarkably
rapid, for often the dominants change every year for several years.

STABILIZATION AND CLIMAX

All successional trends lead toward relative mesophytism within
a climatic area. This explains why related successions parallel each

224 THE STUDY OF PLANT COMMUNITIES • Chapter IX

other in their mature or late stages. Eventually, all successsional
trends lead to a single community, which is composed of the most
mesophytic vegetation that the climate can support and whose mois-
ture relations are average, or intermediate, for the region as a
whole. This community, determined by the climate, terminates
succession and is called the climax community or climax for that
climatic area. It is capable of reproducing itself, and, since it rep-
resents the last stage of succession, it cannot be replaced by other
communities so long as the climate remains the same. It is, there-
fore, a stable community in which the individuals that become
overmature and die are replaced by their own progeny, leaving
the character of the community unchanged.

Uniformity and Variation of Climax.— Since climax is determined
by climate, the distribution and range of a particular climax should
be an indication of a region in which effective climatic factors are
equivalent. Climax is a product of all the interacting factors of cli-
mate and is, therefore, a better expression of the biological effec-
tiveness of climate than man can obtain by physical measurements,
which he must interpret. This is well illustrated by the similarity
of prairie vegetation over an area with an extremely wide range
of several factors, particularly of temperature from north to south.

On this basis, it might be assumed that a climax would be uni-
form throughout its extent. This is true only in part. Certain
variations are to be expected, which are related to the great extent
of climax regions and the history of different parts of these re-
gions. The extent of deciduous forest climax results in transitions
to both coniferous forest and grassland. These transitions are not
abrupt, and the composition of the climax community is affected
for some distance. The deciduous forest likewise illustrates how
the time element may be involved in variation. Most of its north-
ern extent lies on glacial soils and topography and has occupied the
area only in relatively recent times. Unglaciated areas to the south
supported deciduous forest throughout the period of glaciation
and still do today. Thus there are differences in age of vegetation,
topography, and soils, all of which contribute to variation in the
deciduous climax.47

The obvious uniformity of vegetation in a climax region is in
the life form of the dominants, which is definitely a product of

PLANT SUCCESSION 225

climate. Thus the major climax regions are easily recognized :
grassland, desert, and semidesert with shrubs predominating; and
forest climaxes that are boreal, deciduous if temperate, or broad-
leaved evergreen if tropical. In addition to life form there is uni-
formity of genera among the dominants of a climax. Variations of
the dominant species, as well as dependent ones, are a product of
the environmental variations discussed above.

The major climaxes are distinguishable on the basis of physi-
ognomy or life form of the dominants alone. Such climaxes are
termed formations.™ Floristic variation within a formation is usu-
ally sufficient to produce two or more recognizably distinct cli-
max communities, which, following Clements, would be called
associations. Although distinct, the associations of a formation are
at the same time bound together by one or more species present in
all associations and by the constant presence of some dominant
genera throughout. Thus the associations of a formation are quite
obviously similar and related.

Just as associations are recognizable subdivisions of formations,
there are distinguishable variations within associations. These geo-
graphical variants that make up the association are called facia-
tions.60 They are recognizable by differences in the abundance or
relationships of the dominants. Faciations may be further subdi-
vided into local variations, called lociations. Further subdivision is,
of course, possible and often desirable. The various systems of
classification and the terminologies that have been used make for
more detail and controversy than can be presented here.

Because, unfortunately, the term, association, is constantly used
in more than one sense, it deserves further mention. The systems
of classifying communities, as supported by the various schools
of thought, almost invariably include the term. Although not
always in agreement among themselves, European ecologists con-
sistently consider associations as basic units of classification that
can be grouped into categories of successively higher rank. Thus
lociations, as mentioned above, might be given associational rank
in such a system. The use of the term here is in an absolutely
contrasting sense in that it makes it a community of the highest
rank, inclusive of, and divisible into, numerous lesser categories.
It has been suggested that, to avoid conflict, the use of the term

226 THE STUDY OF PLANT COMMUNITIES * Chapter IX

in this sense be indicated by referring to climax associations or
major associations, but this has not been generally accepted as yet.
An attempt was made to standardize the use of the term at a recent
International Botanical Congress, but, even so, the rulings have not
been completely accepted. For a summary of some of the diverse
points of view and some applications of the term, reference should
be made to Conard's67 discussion of plant associations and its ap-
pended bibliography.

Types of Climax.— In a climatic area, all succession is in the di-
rection of a community that can maintain itself permanently, and
there is only one such community for the region as a whole. How-
ever, succession is often halted temporarily in almost any stage of
its progress, and sometimes is halted almost permanently in late
stages. Diseases, fire, insects, or man may produce conditions that
prevent completion of succession and hold it indefinitely at some
stage preceding the climax. Edaphic or physiographic conditions
may be such that succession cannot proceed to completion. Al-
though such communities may appear to be as stable and perma-
nent as climax, they cannot be considered as such because they are
not controlled by climate.

This is the monoclimax hypothesis. In contrast is the polyclimax
view, which recognizes edaphic, physiographic, and pyric cli-
maxes within a climatic area. The conflict between the two views
lies in the interpretation of the concept of climax. Actually, the
same communities are recognized by both but under different
terminology. Since the basic concept of climax implies one ultimate
community controlled by climate, the monoclimax view is con-
sistent with the meaning of the term. When used in conjunction
with a few precise terms,60 which are discussed below, it is ade-
quate for explaining all climax variations.

Subclimax.— When, in any succession, a stage immediately pre-
ceding the climax is long-persisting, for any reason, it can be called
subclimax. It may be the result simply of extremely slow devel-
opment to climax, or of any disturbance, such as fire, that holds
succession almost indefinitely in its subfinal stage. In the eastern
United State, most pine forests are subclimax to hardwood climax
because of the relatively slow elimination of pine in the progression
toward hardwood dominance. In the coastal plain, subclimax pine

PLANT SUCCESSION 227

forests are maintained indefinitely by the constantly recurring
fires to which the pines are resistant and which keep down hard-
woods.

Disclimax.— When disturbance is such that true climax becomes
modified or largely replaced by new species, the result is an ap-
parent climax, called disclimax. The disturbance is usually pro-
duced by man or his animals and the introduction of species that,
under the existing conditions, become the dominants over wide
areas. The prickly pear cactus thus has formed a disclimax over
wide areas in Australia. A grass, Bromus teetotum, forms a discli-
max in much of the Great Basin where, because it burns readily, it
facilitates fires, which reduce dominance of desert shrubs and in-
crease the area of grass. The short grasses of the Great Plains were
long considered as climax but now are generally considered as
disclimax resulting from grazing and drought, which have prac-
tically eleminated the midgrass climax. The ravages of chestnut
blight illustrate how disclimax may result from disease. Oak-chest-
nut climax is today an oak disclimax.

Postclimax and Vr e climax. —Ps. climatic area is normally bor-
dered, on the one hand, by one that is drier and warmer and, on the
other, by one that is moister and cooler. The contiguous climates
are, therefore, either less favorable or more favorable to plant
growth. As a result, each has its own climax, distinct in species and,
often, in growth form. On a large scale, this is apparent in latitudinal
zonation from the tropics to the arctic. Often it is noticeable in the
climaxes along a line from oceanic or maritime climate to the in-
terior of a continent. It is most conspicuous on mountains where
altitude produces a zonation of climates and climaxes. Each of the
climatic areas in such a sequence has a bordering climate with a
more favorable water balance, usually on the north, toward the
coast, or at higher altitudes; while the climate to the south, toward
the interior, or at lower altitudes, usually is less favorable.

For any particular climax the contiguous climax produced by a
more favorable climate, usually cooler and moister, is termed post-
climax, and the one produced by less favorable conditions, usually
drier and hotter, is termed preclimax. To illustrate on a broad basis,
deciduous forest climax has grassland as preclimax and northern
conifer forest as postclimax. At the same time, deciduous forest

228 THE STUDY OF PLANT COMMUNITIES • Chapter IX

holds a postclimax relationship to grassland that has desert as pre-
climax. The use of the concept is not restricted to formations as
illustrated above since it is just as applicable to associations, even
within the same formation. For example, within the deciduous
forest formation oak-hickory is preclimax and hemlock-hardwood
is postclimax to the beech-maple association. Likewise, oak-hick-
ory is preclimax and beech-maple (or hemlock-hardwood) is post-
climax to the oak-chestnut association.

Should the present phase of relatively stable climates be inter-
rupted, the climate of any given area would undoubtedly tend to
become more like that of one of its contiguous areas and a migra-
tion or shift of climax would result. Such a shift occurred during
the glacial period when the northern coniferous forest moved
southward, and the northern extent of the deciduous forest was
proportionately constricted. When the climate ameliorated, the
ice receded, and again, the ranges of the climaxes were readjusted.
When such shifts occur, remnants of the previous dominants are
left behind in locally favorable habitats where they may maintain
themselves indefinitely as relicts of a previous climax. These relicts
are either preclimax or postclimax depending upon their relation-
ship to contiguous climaxes and the direction of the climatic shift.
The habitats in which they survive must have edaphic or physio-
graphic characteristics that differ so markedly from the average
for the region that conditions for growth are similar to those of a
contiguous climatic area. Deep valleys or canyons with steep bluffs
and contrasting exposures, poorly drained flood plains, bogs, ridges
of rock or gravel, areas of deep sand or other peculiar soil condi-
tions are specific examples.

Where there have been shifts of climax, it is apparent that pre-
climax and postclimax communities should occupy such habitats.
Not all preclimax and postclimax communities, however, need be
relicts. Within the general range of a climax, there are bound to be
local habitats such as those mentioned above that will continue
indefinitely to be somewhat more favorable or less favorable, wet-
ter or drier, than the conditions controlled by climate in the region
as a whole. As a result, when vegetational development proceeds
to a condition of stability on such a site, it will have characteristics
of the contiguous more or less favorable climate. Such localized
stable communities are likewise postclimax or preclimax for the

PLANT SUCCESSION 229

region. In or approaching transition zones, such areas are partic-
ularly noticeable, and here, especially, application of the concept
greatly simplifies interpretation of climax.

Since communities such as these exist to some extent in every
climatic area, they must be recognized. As mentioned earlier, not
all ecologists agree as to their interpretation. Some, with the poly-
climax view, describe them variously as edaphic or physiographic
climaxes. This is open to the general criticism that, by definition,
there can be but one climax for a climatic region. Use of preclimax
and postclimax is a necessary part of the monoclimax view but is
consistent with the meaning of climax. At the same time, it shows
relationshiDS with contiguous and past climaxes.

METHODS OF STUDYING COMMUNITY DYNAMICS

Determination of Climax Formations.— The major climax re-
gions (formations) are fairly obvious, and their number and ap-
promixate limits have been accepted for some time. Each has its
distinctive physiognomy or life form that makes for clear demar-
cation. An additional number of criteria corroborating the appar-
ent unity based upon physiognomy have been applied.

Tests of climax that have been used in fixing formations00 are
briefly summarized below. Both static and developmental criteria
must be met.

Static Criteria

1. Life form must be uniform throughout.

2. All associations must include one or more of the same
or closely related species as dominants or subdominants.

Developmental Criteria

3. Late stages of succession must be essentially identical
for a climax; and distinct from those of another climax.

4. Postclimax should show relationships to contiguous cli-
max or subclimax.

5. Historical records as to composition and structure must
conform to the modern picture.

a. Recent historical— old records and land surveys.

b. Historical development reconstructed from pollen
statistics.

c. Geological record, physical history, and fossils.

230 THE STUDY OF PLANT COMMUNITIES • Chapter IX

Recognition of Local Climax.— The variations of a formation
(associations) are not always immediately obvious, particularly in
areas of transition from one association to another. Because of dis-
turbance by man, the climax vegetation once present in virgin
stands over wide areas has practically disappeared. We now, there-
fore, must rely upon small samples of climax vegetation, often
disturbed; or, when even these are lacking, we must determine the
climax on the basis of studies of succession. There may, therefore,
be different interpretations, and errors are possible. To illustrate :
It was generally believed for years that short grasses constituted
the climax of the plains. Added evidence and reinterpretation in-
dicated to many ecologists that mid-grasses are climax and short
grasses are disclimax maintained by modern grazing under the
conditions of periodic drought.

A climax association must, of course, conform to the criteria
that delimit the formation of which it is a member. To check these
criteria, it becomes necessary to know the successional trends of
the vicinity in detail, to know the composition and structure of
the postulated climax and subfinal stages of succession, and to dis-
tinguish preclimax and postclimax communities and habitats. Thus
it becomes necessary to know something of related associations as
well as the one involved. Finally, the history of the region, both
recent and geological, is desirable for proper interpretation of ob-
servations.

The climax must be a community capable of maintaining itself
indefinitely under existing climatic conditions. It must be the final
community in all successional trends in the region except those
isolated instances of edaphic or physiographic variation producing
preclimax or postclimax by compensating for climate. It must
recur throughout the area under average conditions, or the evi-
dence from succession must indicate its potential presence.

General Procedure in Local Study.— The desirability of fa-
miliarity with the area as a whole has been emphasized. Observa-
tion and note-taking should proceed at the same time that literature
is searched to learn the historical aspects of the area and the rela-
tionships of its flora to that of surrounding climaxes. With con-
tinued observation, certain ideas will develop as to probable and
possible successional relationships and the relative position of dif-

PLANT SUCCESSION

231

Quercus alba
0

8 1

\ ■

a t~

\ »

if I

9 / ]
t / J
$ / /

Quercus stellata
D

V \

\ \

B |

\ \
\ 1

A 1

\ 1

\ I

\ \

/' I

// J
It 1

Carya spp
D

BL -L

\ I1

' / 1
/ / /

\ V

Quercus coccinea
D

Quercus borealis
var maxima

Quercus rubra
D

1 V

\ 11

\ M

I /

I I J

u J

Quercus marilandica
D

i !

• i
i I

FlG. 109. A phytographic comparison of the overstory species found in
the two oak-hickory climax variants of the North Carolina Piedmont.183
Values for the white oak type are indicated by solid lines, for the post oak
type by broken lines. D— percent of total tree density, F— frequency percent,
SC— percent of four size classes (overstory, understory, transgressives, seed-
lings) in which the species was found. Zero is the center, 100 percent the
periphery of the circle. Only quantitative data can give information such as
illustrated by these phytographs.

ferent habitats. Such methods alone have produced some excellent
interpretations of vegetational dynamics. General conclusions may
be as good as any obtained otherwise. However, there are reasons
why supporting data are most desirable.

232 THE STUDY OF PLANT COMMUNITIES • Chapter IX

Frequency in Percent

llyrs 22yrs 3 I y r s 3 4 yrj 42yrs 75yr$ 110 yrj Oak -

90 i i Hickory

B Oak and Hickory Trees
\/////A Oak and Hickory Reproduction
[XX^ Pme Trees

Pme Reproduction

25-

20-

* 15-

I 10-

5-
0.

100

290
295

300

Frequency in Percent

Fig. 110. Relationships of trees and reproduction of pine and oak-hickory
in old-field succession in North Carolina as shown by their density and fre-
quency in successive ages of pine dominance leading to oak-hickory climax.
Frequency is indicated by width of columns, density by height. Such phyto-
sociological representations clarify relationships that might otherwise go un-
recognized.

It is often possible for honest observation to be wrong, and only
quantitative and qualitative data will demonstrate the discrepancies.
Again, such data may bring to light pertinent information that
could not be realized by observation alone. When questions of
"why" "when" or "how" come up, they can be most satisfactorily
answered with absolute data.

These things were soon realized by some early students of suc-
cession, and quadrat methods were introduced as a part of their
procedure. Early methods of sampling, however, were rarely ade-

PLANT SUCCESSION 233

quate. Unfortunately, sampling methods in successional studies
were not improved as rapidly as they should have been. Perhaps
students of community dynamics were too much concerned with
an overall picture rather than detail. As a result, much desirable
information was not obtained and now may not be available be-
cause vegetation has been destroyed.

Phytosociological Methods in Studies of Succession.— The
static point of view long held by many Europeans led naturally to
an interest in the detail of community composition and structure.
Sampling methods were an essential part of their work, and, as a
result, these methods were studied and revised for efficiency and
effectiveness. Their objectives and uses were outlined in our dis-
cussion of analysis and description of plant communities. It was for
this purpose that they were developed, but they need not by any
means be restricted to static studies. How successfully they can
be applied to special successional situations is well illustrated by
Billings'20 study of secondary succession and soil changes on aban-
doned fields. It is likewise possible to adapt phytosociological
analytical methods to a comprehensive vegetational study involv-
ing all the major successional trends of a region.183 Herein lies an
application for phytosociological methods that has so far been
given too little attention. In addition to putting on record the
sociological characteristics of the various communities involved,
the same data can be used for clues to solution of stubborn dy-
namic problems, to substantiate observations, and as proof of con-
clusions.

GENERAL REFERENCES

S. A. CAIN. The Climax and Its Complexities.

F. E. Clements. Plant Succession : An Analysis of the Development of
Vegetation.

F. E. CLEMENTS. Nature and Structure of the Climax.

W S. COOPER. The Fundamentals of Vegetational Change.

J. PHILLIPS. Succession, Development, the Climax, and the Complex Organ-
ism : An Analysis of Concepts.

CHAPTER X

THE DISTRIBUTION OF CLIMAX COMMUNITIES
PRESENT DISTRIBUTION OF CLIMAXES

In the early nineteenth century, Humboldt drew attention to
the importance of climate in determining the distribution and
range of species, and Grisebach showed the possibilities of using
communities, instead of species, as units of study. These were the
beginnings of modern descriptive plant geography, which deals
with the extent and distribution of vegetation types, particularly
climaxes, and the reasons they occur where they do. The complex
nature of climate necessitated from the first separate consideration
of its components, and this led to oversimplified explanations of
plant distribution based upon single factors. Even Warming,266 to
whom we are indebted for shaping the foundations of much of
our modern ecological philosophy, was confident that communi-
ties and their responses are primarily controlled by water. Among
the early geographers, Schimper213 deserves special mention be-
cause he emphasized what is now generally recognized, namely,
that a complex of interacting factors determines vegetation. There
is still no simple means of expressing the effectiveness of the com-
plex.

Merriam's173 attempt to correlate all vegetational distribution
with temperature is illustrative of the search for a single factor
whose quantitative value would express climatic conditions. He
showed that zones with similar summer temperature character-
istics frequently have similar vegetation, but, unfortunately, he
assumed that because there was a correlation there must also be a
cause and effect relationship. His generalizations are, therefore, not
acceptable, and too many exceptions remain unexplained.

A persistent search was made by Livingston and his associates159
for a single quantitative value of physiological significance, which,
when plotted to indicate isoclimatic lines, would closely match the
distributions of major vegetation types. Summer evaporation rates,
temperature coefficients, and temperature indices based upon

234

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

235

physiological responses all were tried. The most successfully ap-
plicable value he found156 was one that combined a physiological
temperature index, precipitation, and evaporation. Actually this
was a refinement of the precipitation : evaporation ratio proposed
earlier,255 but it is scarcely more useful. These and other studies
serve to emphasize the complexity of plant-environmental rela-

m TUNDRA

ED BOREAL FOREST

E%1 HEMLOCK-HARDWOOD F.

ES3 DECIDUOUS FOREST

E-7H3 S.E.EVERGREEN FOREST

Fg~\ GRASSLANDS

r?rl DESERT CRASS AND SCRUB

r^il DESERT

V77ZK COASTAL FOREST

rrrrm rocky mt. forest

E^ WET AND DRY TROPICAL F.

FlG. 111. General ranges of the principal vegetation types of North Amer-
ica.— By permission jrom Transeau, et al (1940)~5: Harper and Brothers, pub-
lishers.

236 THE STUDY OF PLANT COMMUNITIES * Chapter X

tionships and the impracticality of expressing them as a function
of a single variable. This becomes even more obvious when influ-
ences such as length of day, winter temperatures, and the season
of precipitation are considered.

CLIMAX REGIONS OF NORTH AMERICA

The vegetation maps available for North America230'236'268 serve
to emphasize by their similarities that the major vegetation types
are fairly obvious, but their differences in detail indicate disagree-

FlG. 112. Alpine tundra in the Colorado Rockies— U. S. Forest Service.

ment on the interpretations of climax relationships, especially
within formations. An understanding of the bases for different
interpretations can best be obtained by study of the many papers
dealing with local investigations of vegetation. There are, how-
ever, several of a more comprehensive nature,230' 231> 118 which give
more detail than can be presented here.

The concept of climax formations and associations was discussed
earlier (Chap. 9). Classification of North American vegetation on
this basis is altogether logical, particularly if the point of view is a
dynamic one. The system shows modern successional and climatic
relationships but is based as well upon past history of the climaxes.
Although growth form is the apparent major basis of classification,
dynamic factors are given equal consideration.

Below are listed the major climax formations of North America.

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 237

These, together with their associations, are discussed in the section
that follows. The formations restricted to the mountains of the
west occur in altitudinal zones whose relationships should be clear-
ly understood. Consequently, discussion of these zonal formations
is centered about each of the principal mountain ranges rather
than considering each zone separately, throughout its extent.

Climax Formations of North America
Tundra

Tundra Formation

Forest

Coniferous

Boreal Forest Formation

Subalpine Forest Formation

Montane Forest Formation

Pacific Coastal Forest Formation
Deciduous

Deciduous Forest Formation
Woodland

Woodland Formation
Scrub

Broad-Sclerophyll Formation

Sagebrush Formation

Desert Scrub Formation
. Grassland

Grassland Formation
Tropical Formations

Tundra Formation.-Tundra lies between the northern limit of
trees and the area of perpetual ice and snow in the far north, or
above timber line in high mountains. In North America, it forms
a broad band completely across the continent, and it also occupies
the narrow low coastal area around most of the periphery of
Greenland. It occurs on mountains as far south as Mexico if their
altitude is sufficient to produce a timber line. Thus it is limited in
its northern or upward extent by ice and bounded on the southern
or lower margin by boreal or subalpine conifer forest.

Vegetation is low, dwarfed, and often matlike, and includes a

238 THE STUDY OF PLANT COMMUNITIES • Chapter X

high proportion of grasses and sedges. Even the woody plants, in-
cluding willows and birches, are prostrate. The herbs are mostly
perennial and of a rosette type, producing relatively large flowers,
often with conspicuous colors. A4osses and lichens may grow any-
where and in favorable habitats form a thick carpet with the low
herbs. The number of species is small compared with floras of
temperate climates, and, even within the tundra, the number de-
creases northward. Most of the genera and numerous species are
to be found throughout the Northern Hemisphere wherever
tundra occurs.

The uniformity of the flora is undoubtedly related to the pe-
culiarities of environment. The growing season is short and its
temperatures are relatively low. The depth to which soil thaws
in summer is of great importance. Light is continuous throughout
the growing season in the arctic, and is intense and high in ultra-
violet rays in alpine habitats. Precipitation is largely in the form
of snow and varies greatly. Drying summer winds, which are
characteristic, produce high rates of evaporation and transpira-
tion. As a result, water is often a critical factor, especially inland
away from moist coasts. Local marked differences in vegetation
are commonly related to minor variations in topography and the
differences they produce in drainage and retention of snow. The
poor, haphazard drainage associated with new topography is ap-
parent everywhere.

Arctic Tundra— Although the flora of the tundra is fairly well
known, its communities and their successional relationships have
not been sufficiently studied.20* In contrast with temperate vege-
tation, many species may occur in any type of habitat, and several
that appear to be climax may also be pioneers in the newest of
habitats. Even climax is not agreed upon, possibly because observa-
tions have been made at widely separated points. Interpreted in
terms of Greenland vegetation, Cassiope heath appears to be cli-
max, and a Sedge-Dryas dominated community, of equal extent
but on drier sites, is preclimax.185 Two subclimaxes are frequent.
Any habitat with sufficient moisture, whether it be pond margin,
seepage area, or boggy ground, eventually is covered with a thick
moss mat supporting several herbs of which cotton grass (Eri-
ophorum spp.) is most conspicuous. Xerarch succession on rock

CLIMAX COMMUNITIES: PRESENT DISTRIBUTION 239

exposures eventually results in a lichen-moss mat, which may con-
tinue almost indefinitely.

Important climax dominants are Cassiope tetragona, one or more
species of Vaccinium, Arctostaphylos alpina, Empetrum nigrum,
Andromeda polifolia, Ledum palustre, Rhododendron lapponicum,
and species of Betula and Salix. These and other species occur in
varying combinations and degrees of importance.

Practically all habitats support some of the many species of
Carex, of which the commonest include Carex capillaris, C. nar-
dina, and C. rupestris. The preclimax sedge community invariably
includes Elyna bellardii in abundance. Some grow in mats, some
are in clumps, but all are dwarfed. The same can be said for the
grasses, which, although relatively abundant and widespread, are
restricted to a few genera, of which Festuca and Poa are espe-
cially well represented. Many of the conspicuous herbs previously
mentioned are included in the numerous species of one of the
following genera : Saxifraga, Potentilla, Ranunculus, Draba, Cer-
astium, Silene, Lychnis, Stellaria, Castilleja, and Pedicularis. Con-
spicuous and widespread species typical of tundra are Oxyria
digyna, Papaver spp., Dry as octopetala, and Epilobium latifolium.

Alpine Tundra— Mountains high enough to have a timber line
support tundra, whose upward extent is limited by the snow line.
In the east, as a consequence, tundra is found only on a few high
peaks in New England. Farther south, the Appalachians are not
of sufficient height to support tundra. That on Mt. Washington
is representative of the type and is essentially similar to the not far
distant arctic vegetation.

Alpine tundra in the western mountains mostly lies far to the
south of the arctic and is consequently found at high altitudes
only. In the Canadian mountains, it is found as low as 6,000 feet,
but southward its altitudes grow progressively higher. In the
Rocky Mountains of Colorado, it is well developed between
11,000 and 14,000 feet. In the Sierra Nevada, where many peaks
are higher, the snow line is lower, and thus, tundra lies mostly
between 10,500 and 13,000 feet.

When climate changed and terminated the glacial period, vege-
tation similar to modern tundra must have followed the ice as it
receded northward. This left only these high peaks and ridges

240 THE STUDY OF PLANT COMMUNITIES • Chapter X

where tundra could survive as relicts. The relict vegetation obvi-
ously belongs to the Tundra Formation because of the growth
form and the duplication of characteristic genera as well as many
species. The greater importance of grasses and the presence of
numerous endemics in the western mountains suggest that both
the Sierran and Petran tundras might be classed as associations of
the Tundra Formation.

Boreal Forest Formation.— This great forest, often called "taiga"
in its northern extent, spans the continent in a broad band to the
south of the tundra. Along the Atlantic coast it extends from
Newfoundland on the north to the New England states on the
south. Westward, the southern boundary touches the Great Lakes
region, trends northwestward across Saskatchewan and along the
Rocky Mountains, and then to the Pacific coast in Alaska. The
band is, therefore, narrowed abruptly in the far west although it
extends much farther to the north there than it does over much of
the continent.

Climate is scarcely less severe than that of the tundra. The short
growing season from June through August is cool, and winters
are very cold. Precipitation is moderate, averaging perhaps twenty
inches, except on the east coast where it may be forty inches. The
precipitation : evaporation ratio is, however, favorable because of
the low temperatures. The topography is almost entirely that pro-
duced by glaciation. Lakes are scattered everywhere, and many of
them have filled to form extensive bogs or muskegs. The mineral
soils are either thin and residual, overlying the rock masses ex-
posed by glaciation or, along the southern boundary, deep moraine
and outwash. All are immature and often poorly drained. Sub-
soils, in the bogs especially, may not be frost-free even in mid-
summer.

Climax— The climax forest of white spruce and balsam fir is
best developed in and about the St. Lawrence river valley where
the trees reach maximum size and grow in close stands under a
variety of conditions. Here, and over much of the range, Picea
glanca and Abies balsam e a form dense stands under whose canopy
there are relatively few dependent or secondary species. Paper
birch (Be tula papyrifera) is a constant associate although it is
successional after fire or disturbance and often occurs as subclimax

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 241

in pure stands. Characteristic tall shrubs are Viburnum alnifolium
and V. cassinoides. Typical lesser plants on the shady forest floor
are Aster acuminatum, Dryopteris dilatata, Oxalis montana, Clin-
tonia borealis, Cornus canadensis, Maianthemum canadense, Aralia
nudicaidis, Coptis trifolia, and Chiogenes hispidula.

With increasing distance from the St. Lawrence center, both
westward and northward, the number of species declines. Balsam

FlG. 113. Interior of boreal white spruce-balsam fir forest as it appears in
northern Michigan.— U. S. Forest Service.

fir is completely absent along the northern boundary and in most
of the western range of the type. Beyond the range of fir, the
subclimax species, otherwise found in bogs or on burned areas,
often appear with white spruce as climax. Along the northern
transition tamarack (Larix laricina) may take an essentially climax
position as does the black spruce (Pice a mariana), especially on
high rocky ground. Both are bog species farther south. To the
west, paper birch and jack pine (Finns banksiana) have climax
characteristics although both are definitely subclimax nearer the
center.

Successions.— Primary succession occurs mainly on bare rock
or ir lakes.70 The former is initiated by xerophytic mosses and

242 THE STUDY OF PLANT COMMUNITIES * Chapter X

lichens, which, after mat formation, lead to a heath mat stage. In
the western part of the range, this is followed by the xerophytic
jack pine, or black spruce to form a subclimax, but eastward white
spruce-balsam fir may come in directly. Jack pine also occupies
extensive areas of sand plains and gravelly soils.

Fig. 114. Typical stand of jack pine (Pinus banksiana) on sand or gravel
soils in northern Michigan.— U. S. Forest Service.

Bog succession is everywhere apparent in the many lakes that
are filling up. The usual submerged and floating-leaved aquatics
are commonly followed by sedges and grasses, which may form
a floating mat upon which a bog-shrub stage develops. This may
include Chamaedaphne calycidata, Andromeda polifolia, Almis in-
cana, Ledum groenlandicmn, and Vaccinium spp. Larch is the
commonest tree to come in after shrubs, followed by black spruce
or, in less acid bogs, sometimes Thuja occidentalis. Any of these
species may maintain their dominance for long periods, but they
can be superseded by climax.

Secondary succession is usually caused by fire. If the burn is so
severe that all humus is consumed, leaving bare rock, primary suc-
cession may be repeated. If a dry peat bog burns, it usually fills

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

243

with water again, and succession is reinstated at the aquatic stage.
More often a burn results in pure stands of paper birch, which
eventually give way to climax. Wind throw and lumbering of
climax stands may also result in birch or aspen dominance but
sometimes are followed directlv by climax species.

FlG. 115. Aspen stand (Populus tremuloides) at forty-five years of age in
northern Alinnesota. Its successional nature is clearly shown by the well-
developed understory of spruce and fir.— U. S. Forest Service.

Transitions— The, northern border is abrupt, but the line is ir-
regular depending upon topography. Forest extends far into the
tundra in sheltered valleys, and tundra appears on the high ridges
well within the forest area. Timber line seems to be advancing in
Alaska, retreating in eastern Canada, and remaining more or less
stable in the interior. The southern transition is to deciduous for-
est in the east and to grassland in the west. From New England to
Minnesota, the transition is marked by pure stands of white pine
(Finns strobns), a subclimax of long duration. In the lake states
red pine (P. resinosa) and jack pine may also occupy similar po-
sitions on less favorable sites. Scattered individuals of white pine
especially tend to persist well into the climax. Through much of
the eastern transition, spruce, fir, and hardwoods may grow in

244 THE STUDY OF PLANT COMMUNITIES * Chapter X

mixture or in alternating stands. The transition to grassland in the
Middle West is marked by aspen (Populus tremuloides)llb in a
band some fifty miles wide. In spite of fluctuations produced by
fire, grazing, and drought, the trees persist and, in some instances,
seem to have advanced into the grassland. In the west, along the
Rockies, the subalpine Abies lasiocarpa is associated with Pice a

FlG. 116. Interior of red spruce-Fraser fir forest in the southern Appala-
chians. Compare with Fig. 113.— U. S. Forest Service.

glauca, and northward in Alaska there is a merging with the
northwestern coastal forest.

Appalachian Extension — -On the higher mountains of the Ap-
palachian system, the northern conifer forest extends as far south
as the Great Smoky Aiountains of North Carolina. The growth
form and associated species are in every way similar to the main
body of the formation, but, from New Brunswick southward into
New England, red spruce (Picea rubens) tends increasingly to re-
place white spruce. Still farther south, Fraser fir (Abies fraseri)
takes the place of balsam fir so that the dominants in the southern
Appalachians are ecologically equivalent to those elsewhere in the
formation but are taxonomically distinct. It seems reasonable to
consider the Appalachian extension as a distinct association whose
limits are marked by Picea rubens. A northern and southern facia-

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 245

tion are suggested by the presence of Abies balsamea and Be tula
papyrifera in the north but the substitution for them in the south
of Abies fraseri and Betula lutea.

The compensating effect of latitude is apparent in the altitudinal
limits of the association, which increase southward. In the north-
ern range of red spruce, it may be found anywhere, as is true of

7. Mixed hardwood forest in Indiana. Large trees are white oaks.—

U. S. Forest Service.

fir. Southward, the approximate lower limit of spruce-fir forest on
Alt. Katahdin is 500 feet; in the White Mountains, about 2,500
feet; in the Adirondack Mountains, 3,000 feet; in the Catskills,
3,500 feet; and in the Great Smoky Mountains, almost 5,000 feet.
Deciduous Forest Formation.-This formation occupies all of
the eastern United States except southern Florida. Its northern
transition to conifer forest extends into Canada along a line from
northern Minnesota to Maine. On the west, forest gives way to

246 THE STUDY OF PLANT COMMUNITIES • Chapter X

Grassland as precipitation : evaporation ratios become less favor-
able. The irregular line of transition runs northward from eastern
Texas with thirty-five inches of precipitation, to central Minne-
sota where precipitation falls to twenty-five inches.

The great extent of the deciduous forest includes soils and to-
pography of diverse nature and origin. The northern portion was

FIG. 118. Sugar maples (160-200 years old) in beech-maple forest associa-
tion, Pennsylvania— U. S. Forest Service.

glaciated. There are mountains in the east. The great valleys of
the Mississippi and Ohio Rivers are included as are the Piedmont
Plateau and coastal plain of the Atlantic and Gulf coasts. Any and
all kinds of topography as well as soil types are, therefore, repre-
sented.

Climate is temperate with distinct summer and winter, and all
parts are subject to frost, one of the few environmental factors
that applies throughout. Precipitation varies from sixty inches in
the southern mountains to less than thirty inches northwestward,
but it is everywhere fairly well distributed throughout the year.
The ratio to evaporation is most favorable in the north, the east,

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 247

and in the mountains and becomes decreasingly favorable ap-
proaching the transition to prairie.

The southern Appalachians represent the oldest exposed land
surface in the region. Here the deciduous forest is more complex
than in any other part. Practically all of the species found else-
where in the deciduous forest are represented, as well as several

FlG. 119. Sugar maple-basswood forest, illustrating the climax for much
of southern Wisconsin and Minnesota— U. S. Forest Service.

others. Numerous endemics occur as associates. Most of the trees
also attain their greatest size here. Away from the mountains, the
number of species declines, and habitat requirements become of in-
creasing importance. It is believed that a forest similar to the pres-
ent one has existed here since Tertiary time. Such evidence is
taken to mean that the southern Appalachians are a center of origin
for much of the widespread deciduous forest. The distribution and
nature of the several associations of the formation give additional
supporting evidence. In general, with increasing distance from
the center, the associations are made up of fewer species and yet
all are bound together or interrelated by several species that range
throughout.

Mixed Mesophytic Forest Association.-Thtonghoxit the Ap-

248 THE STUDY OF PLANT COMMUNITIES * Chapter X

palachian and Cumberland plateaus, the numerous species of this
climax grow in varying combination. Fagus grandifolia, Aes cuius
octandra, Magnolia acuminata, Tilia spp., Liriodendron tidipifera,
Acer saccharum, Quercus alba, and Tsuga canadensis are the most
abundant trees, but there are twenty or twenty-five other species,
any of which may have climax status. The differing sensitivity of

FlG. 120. Seventy-year-old jack pine with a strong understory of balsam,
indicating the trend that succession may take in the Lake States region.—
U. S. Forest Service.

the species to minor variations in environment result in their oc-
currence in all kinds of combinations, which may be referred to
as association-segregates.32 The best indicators of the association
are large trees of basswood (Tilia heterophylla) or buckeye (Aes-
culus octandra).

The association prevails in the Cumberland and southern Al-
legheny mountains and in the adjacent Cumberland and Allegheny
plateaus.33 Away from this center, there is a progressively increas-
ing tendency toward restriction to the most favorable habitats. To
the south, the association is seldom found except in the moist coves

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

249

of the high Appalachians. To the west, southwest, and east it is
found only in ravines and deep valleys. To the northwest, it is
represented in southern Ohio by a mixed hardwood forest of far
fewer species.

Beech-Maple Association— The northward extension of the
mixed mesophytic forest shows an increasing importance of beech

FlG. 121. Virgin white pine (Pinus strobus) forest in Connecticut, of the
type that once occurred over wide areas in the northeast.— U. S. Forest Serv-
ice.

(Fagns grandifolia) and sugar maple (Acer saccharum). North of
the boundary of Wisconsin glaciation, they are the climax species
over an area west of the Alleghenies from New York to Ohio and
up into Wisconsin.31 Virgin forest in Michigan showed beech pre-
dominating over maple, and associates included red maple (A.
rubrum), elm (Ulmns america?ia), red oak (Quercus borealis var.
maxima) and black cherry (Primus serotina).^ The original for-
ests of southwest Michigan, as reconstructed from land survey
records, were beech-maple on good sites and oak-hickory on
coarse soils with poor moisture conditions.139 This conforms with
present conditions and can be interpreted as climax and preclimax.
Maple-Basszvood Association- -The natural range of beech does

250 THE STUDY OF PLANT COMMUNITIES ' Chapter X

not extend to the northwest limits of the deciduous formation.
Beech is replaced in the climax by basswood (Tilia americana),
beginning in Wisconsin and continuing into Minnesota.95 Other-
wise the community is changed very little.

Hemlock-Hardwoods Association — between the northern con-
iferous forest and the deciduous forest lies a transitional association

&.L2

FlG. 122. Virgin hemlock (Tsuga canadensis) as it once occurred in the
hemlock-hardwoods association of the northeast and in mountain coves
southward.— U. S. Forest Service.

of which hemlock (Tsuga canadensis) is an important and con-
stant member, together with beech and sugar maple, and, in lesser
numbers, yellow birch (Betida Intea), white pine, basswood, elm,
white ash (Fraxinus americana), red oak, and other species. The
association, which extends from northwestern Minnesota through
the Lake States to Nova Scotia, has been given various names by
authorities with different points of view. It is the area throughout
which occurred the magnificent pine forests of the recent past—

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

251

now mostly decimated by fire and lumbering. Where pine was
dominant, Finns strobus tended to occur on sites with more favor-
able moisture conditions than the sand plains and ridges occupied
by P. resinosa. By some268 these pure stands of pine are considered
to be climax, but many more ecologists agree that the pines are
successional species occupying inferior sites for long periods as

FlG. 123. The oak-chestnut forest that once occupied the lower slopes of
much of the Appalachian system.— U. S. Forest Service.

subclimax. That white pine especially carries over into the hard-
wood climax180 is undoubtedly true. Its long life and relatively low
numbers suggest that these trees in the climax should be regarded
as relicts even though they can maintain their numbers by repro-
duction under openings appearing in the hardwood canopy.161

Postclimax forests of the northern conifers— tamarack, black
spruce, white cedar (Thuja occidentalis)— occupy the many bogs
throughout the area. The extensive areas denuded by lumbering
and fire are today largely occupied by second-growth forests of
aspen or pine.

Oak-Chestnut Association.— As the mixed mesophytic forest
becomes restricted to special habitats to the east and southeast of
its center, the slopes and uplands are occupied by what was, until

252 THE STUDY OF PLANT COMMUNITIES • Chapter X

recently, oak-chestnut forest. The almost complete elimination
of chestnut (Castanea dentata) by blight has left practically none
of the original forest that extended along the mountains from
southern New England to Georgia. Chestnut oak (Quercus mon-
tana) and scarlet oak (Q. coccinea) are important species today.
Tulip poplar, red and white oaks, and some hickory are common

FlG. 124. Savannah-like transition from deciduous forest to grassland. Bur
oak predominates in these scrubby clumps of trees on the Anoka sand plain
northwest of Minneapolis. Note blowout in sand dune in process of restabili-
zation by Hudsonia— Photo by W. S. Cooper.

associates. None of this association remains in its original state
today, for the remnants untouched by extensive lumbering opera-
tions have been modified by the ravages of chestnut blight.

Pitch pine (Finns rigida) is the important successional species
throughout the range, but shortleaf and Virginia pine (P. echinata,
P. virginiana) are increasingly noteworthy southward.

In its southern extent, the association is restricted to the moun-
tains, occupying most of the favorable slopes. Northward it is
found on progressively lower sites, occurring as far east as Long
Island.66 Through the foothills of the mountains, it grades into the
oak-hickory climax of the bordering Piedmont Plateau.

Oak-Hickory Association— In all directions from the deciduous
forest center, except northward along the mountains, precipitation
decreases and becomes less effective. This results in dominance by

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

253

the drought-resistant oak-hickory association, which consequently
occurs as a fringe around all the margin of the formation except
toward the north. Oak-hickory climax ranges through much of
the Piedmont Plateau and the Atlantic and Gulf states coastal plain
in an arc that widens westward to eastern Texas. North from east-
ern Oklahoma it may become savannah-like where it grades into
prairie, but it is more or less continuous to western Minnesota.

Fig. 125. Typical longleaf pine savannah (Georgia) as maintained by al-
most annual burning. Note that the only apparent ground cover is wire grass
(Aristida), which is an important factor in facilitating fire— U. S. Forest
Service.

Northwest of the Appalachian center, in unglaciated parts of
Ohio and Indiana, oak and hickory occur in combination with
numerous other species, forming .a mixed mesophytic forest cli-
max, which suggests, by its similarity, that the mixed mesophytic
association may still be expanding its range. Throughout the asso-
ciation, various combinations of oak-hickory may occur as pre-
climax. Postclimax communities of mixed forest may be found
within the oak-hickory area on sites, such as old flood plains, where
moisture may be exceptionally favorable.183 Beech, sugar maple,
willow oak (Quercus phellos), overcup oak (Q. lyrata), swamp
chestnut oak (Q. prinus), and shagbark hickories are indicator
species.

254 THE STUDY OF PLANT COMMUNITIES * Chapter X

The dominants of oak-hickory forest are not the same through-
out its extensive range, but several species occur consistently.
Quercus alba, Q. bore alls maxima, Q. velutina, Q. stellata, Q.
marilandica, Carya cordijormis, C. ovata, C. alba, and C. laciniosa
are species that may be found in the climax anywhere. Other oaks
and hickories with more restricted ranges may be in association

FlG. 126. Slash pine savannah after protection from fire for only a few
years. With continued protection, the pine will soon form a closed stand
with shrubs and hardwoods forming an understory— U. S. Forest Service.

and produce local variations. Shingle oak (Q. imbricaria), not so
important in the east, should be added for the western forest from
Arkansas and eastern Oklahoma37 northward.4 Bur oak (Q. ma-
crocarpa) is the characteristic tree of the sometimes extensive sa-
vannah-like transition from forest to grassland, as well as along
the rivers in the prairie, from Texas to Minnesota. Constant sub-
ordinate species are sourwood (Oxydendrum arbor eum), dog-
wood (Cornus florida), black gum (Nyssa sylvatica), and sweet
gum (Liquidambar styraciflua).

Because of the amount of abandoned land throughout the east-
ern and southern range of the association, old field succession is

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 255

particularly noticeable, and subclimax pine stands are conspicuous
(see Figs. 108 and 110). Virginia pine (Finns virginiana) predom-
inates in the northern Piedmont, but southward and westward
shortleaf (P. echinata) and loblolly pine (P. taeda), usually in pure
stands, precede the climax in secondary succession on uplands.
Successional trees in lowlands are sweet gum, tulip poplar (Lirio-

FlG. 127. Scrubby, open oak forest (mostly 0- catesbaei and Q. cinerea)
of the southeastern sandhills areas. The open stand and expanses of bare
white sand are typical.— Photo by H. L. Blomquist.

dendron tulipifera), sycamore (Platanus occidentalis), river birch
(Betida nigra), red maple, elms (Ulmus spp.), ash (Fraxinus spp.)
and hackberry (Celtis spp.).

Fire and Swamp Subclimaxes of the Coastal Plain— The coastal
plain, once covered by the sea, extends from New Jersey down
into Florida and along the Gulf to Texas as a low-lying, relatively
level area, mostly overlayed with sandy soil. Drainage is poor, re-
sulting in much swampy ground, but any raised area between
streams is apt to be very dry for a part of each year. The height
of the water table during the wet seasons and the amount of fire in
dry seasons are fundamental factors in determining the nature of
the vegetation.

From the pitch pine barrens of New Jersey through loblolly
pine and longleaf and slash pine in the more southern states, fire

256 THE STUDY OF PLANT COMMUNITIES ■ Chapter X

maintains pine dominance, usually in open stands, called savannahs,
with the highly combustible wire grass (Aristida stricta), for
ground cover. These stands owe their origin and maintenance to
their resistance to fire.53a If protected from fire, they would un-
questionably be replaced by oak-hickory dominated forest.269 No
extensive areas exist where fire has been excluded for more than a

FlG. 128. Interior of a Florida hammock.

relatively few years. The successional evidence is clear enough,
however, and pine in the coastal plain must therefore be classed as
a fire-maintained subclimax within the oak-hickory association.

A possible preclimax is the scrub oak-hickory forest found on
sand dunes near the coast and inland. Turkey oak (Quercus cates-
baei), margarete oak (Q. margaretta), blue jack (Q. cinerea) and
black jack oak (Q. marilandica) are dominants. Wire grass may
be present, but often the sand is bare, glaring white in the sun,

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 257

except for a few characteristic herbs. These include Euphorbia
ipecacuanhae, Jatropha stimulosa, Stipulicida setacea, Polygonella
polygama and Selaginella acanthonota.272

Undrained, shallow depressions in savannahs form upland bogs
or pocosins, sometimes acres in extent, in which evergreen shrubs
predominate. Ilex glabra, Myrica cerifera, Cyrilla racemiflora,

Fig. 129. Southern white cedar bog (Chamaecy parts thyoides) in New-
Jersey. Note well-drained site.— U. S. Forest Service

Persea borbonia, P. pubescens, Magnolia virginiana, and Gordonia
lasianthus are representative of the numerous tall shrubs or small
trees. With them are usually a large number of ericaceous shrubs
of smaller size. All are commonly overgrown with lianas, of which
Smilax laurifolia is most abundant. The presence of Pinus rigida
serotina in the bogs explains its name of pocosin pine. Sphagnum
is the usual ground cover.

It is at the margins of pocosins and in wet savannahs in North
Carolina that the venus fly trap (Dionaea muscipirta) is found,
sometimes in great abundance but never continuously over an ex-
tensive area. With it several other insectivorous plants may occur.
Species of Sarracenia, Drosera, and Pinguicula are common.

The hammocks of Florida, in contrast with pocosins, are mesic

258 THE STUDY OF PLANT COMMUNITIES ■ Chapter X

habitats raised somewhat above surrounding wetter areas. Over
much of Florida their dominants suggest postclimax to oak-hick-
ory, but toward the southern tip of the state, the species are more
and more subtropical.

Any shallow depression in the flatland of the lower coastal plain
fills with water. Permanent standing water results in open

FlG. 130. A4aritime live oak forest (Quercus virginiana) on Smith's Island,
N. C. Once characteristic of the banks and islands of the south Atlantic and
Gulf Coast, much of it has been destroyed because of neglect. Note the
dunes at right, which were once forested.— Photo by C. F. Korstian.

marshes,198 sometimes miles in extent, dominated by rushes and
grasses. If flooding is not continuous, subclimax swamp forests de-
velop. Bald cypress (Taxodium distichum), which dominates
where water normally stands most of the year, occupies stream
and lake margins or entire lakes to the exclusion of other trees.
Gum swamps are usually flooded only seasonally. Nyssa bi flora
and Nyssa aquatica are the important species,114 with ash (Fraxinus
profunda, F. caroliniana), bald cypress, and red maple as associates.
The less the flooding, the greater the number of pocosin species
that may be present.16

Still another forest of undrained areas is formed by Chamaecy-
paris thyoides, which occurs on peat bogs where it apparently be-
comes established only after fire occurs when the water table is

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 259

high. Although the stands have subclimax characteristics, there is
evidence that they may be succeeded by species characteristic of
pocosins.38 These valuable trees have been cut so systematically
that they remain only as small sample stands or in relatively inac-
cessible places.145

Perhaps the most extensive bog and swamp forests still remain-
ing in virgin condition are to be found in parts of the Dismal
Swamp in Virginia and in the Okefenokee Swamp in Georgia.

The plant communities of the banks155 and islands197 along the
coast, as well as a narrow fringe of the coast itself, are distinctive
enough to merit more discussion than can be given them here. The
effects of salt spray on vegetation were considered earlier, (p. 102.)
Live oak (Quercus virginiana) is the most important tree of the
forested areas, and the associated shrubs include Myrica cerifera,
Ilex vomitoria, Batodendron arboreum, and several others, mostly
evergreens.199' 271 Thus, this maritime climax forest is an evergreen
variant of the oak-hickory association.

Rocky Mountain Forest Complex.— Changes of environmental
factors with altitude and the resulting zonation of vegetation on
mountains have been discussed earlier (see Fig. 66 and related
text). The great height of the Rocky Mountains provides condi-
tions for a discontinuous alpine zone on the peaks, a subalpine
zone, a montane zone, and a zone of woodland forest, which grades
into the surrounding desert or grassland. These zones are recog-
nizable by their distinctive climax vegetation over an area extend-
ing latitudinally from northern Alberta to the southern end of the
Sierra Madre of northern Mexico and from the Black Hills of
South Dakota on the east to the eastern foothills of the Sierra
Nevada and the eastern slopes of the Cascades on the west.

Climaxes with so great an areal extent would be expected to
vary somewhat in different parts of their ranges, especially as to
associated species. The zones are not always continuous, nor are
they always all present. Near the northern limits of the area, the
lower zones run out and the upper zones are found at relatively
low altitudes. Southward all zones are, of course, found at succes-
sively higher altitudes. Because the prevailing winds are from the
west and carry with them oceanic climatic influences, the entire
eastern slope of the Rocky Mountain system has different growing

1 J J o t>

260 THE STUDY OF PLANT COMMUNITIES ■ Chapter X

conditions from those of the west slope and, accordingly, dif-
ferences in vegetation. Within the system, the individual ranges
likewise have similar east-west slope differences. North and south
exposures produce marked irregularities in zonation. Narrow val-
leys permit the dominants of one zone to extend downward into a
lower zone, and high dry ridges allow upward, fingerlike projec-
tions of dominants into continuous higher zones. Cold air drainage
locally causes marked disruption of the zonal pattern.

The factors operative in producing and controlling vegetation
and its zonation in the Rockies have been studied in a number of
localities just as there have been many local studies of the vegeta-
tion. An unusually complete review and synthesis of all these in-
vestigations is available85 with an extensive bibliography. What
follows is largely an adaptation from this report.

Vegetation Zo7ies.—The zonal climaxes may be grouped as fol-
lows :

Alpine Zone

Tundra Climax (discussed earlier— p. 239-240)
Subalpine Zone

Engelmann spruce— Subalpine fir climax
Montane Zone

Douglas fir climax

Ponderosa pine climax
Foothills (Woodland) Zone

Pinon-Juniper climax

Oak-Mountain mahogany climax

Each of these types of vegetation extends over an altitudinai
range of about two thousand feet, where fully developed, and is a
true climax. The foothill zones narrow down and disappear en-
tirely in the north where the upper zones are found at progres-
sively lower altitudes.

Near the upper and lower limits of a zone, the characteristic
species are more and more restricted to special habitats. Upward,
the climax species do best on south-facing slopes, which are warm-
er and drier than the general climate. Thus, in its upper transition
area, each association shows its preclimax relationship to the climax
of the next higher zone. At its lower limits, the association tends

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 261

to be restricted to moist and cool sites and extends into the next
lower zone only in such habitats. It, therefore, holds a postclimax
relationship to the climax below. Subalpine and alpine zones tend
to be drier and colder than the zones below, and, consequently,
preclimax and postclimax relationships may be reversed above the
montane zone.

FlG. 131. Virgin Engelmann spruce (Picea engelmamii), with some alpine
fir (Abies lasiocarpa) of the subalpine zone in Colorado.— U. S. Forest Service.

Subalpine Spruce-Fir Climax— From timber line downward for
about two thousand feet, the climax forest is made up largely of
Engelmann spruce (Picea engelwmnnii) and subalpine fir (Abies
lasiocarpa), which grow in dense stands. The spruce is the larger
and more abundant tree. In Arizona, New Mexico, and southward,
Abies lasiocarpa var. arizonica is as important as A. lasiocarpa. In
Montana and northern Idaho, mountain hemlock (Tsuga merten-
siana) is often found in the zone, and still farther north, approach-
ing the merging with northern conifer forest, Picea glauca and
A. lasiocarpa may grow in association.

Subordinate species vary far more than do the dominants. On
the relatively dry eastern slope of the central Rockies, ground

262 THE STUDY OF PLANT COMMUNITIES • Chapter X

cover is sparse and made up largely of dwarf Vacciniums, while
the moister west slope has an abundance of bryophytes and herbs.
Northward, the bryophytes increase until they practically cover
the ground, and the vascular plants, both herbs and shrubs, also

increase.

The most conspicuous succession in the subalpine zone follows
fire and may result in subclimax stands of lodgepole pine (Finns

FIG. 132. Dense aspen stand (Populus tremidoides) that came in after fire
in the subalpine zone in New Mexico. Spruce reproduction underneath.—
U. S. Forest Service.

contorta var. murrayana), aspen (Populus tremidoides), or Doug-
las fir (Pseudotsuga taxifolia). Progression to climax is extremely
slow. Lodgepole pine is absent in the southern Rockies, but else-
where aspen is favored over the pine on moist sites, and after light
fires it has an advantage, probably because of its ability to regen-
erate from sprouts. Near timber line, burned areas are revegetated
directly by climax.

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

263

The transition from subalpine forest to alpine tundra is usually
gradual with a thinning out of trees, which here commonly have
the dwarfed and distorted form known as Krummholz. Character-
istic of timber line are several trees that cannot survive in the
tundra above and cannot compete with climax species below,
where they are only found on dry and windswept ridges. Foxtail

FlG. 133. Foxtail pine (Pinas aristata) Krummholz at timber line of the
subalpine zone in Colorado— U. S. Forest Service.

pine (Pinus aristata) occupies this position in the southern Rockies,
limber pine (P. flexilis) in the central Rockies, whitebark pine (P.
albicaulis), and alpine larch (Larix lyallii) in the northern Rockies,
except in the far north where lodgepole pine occurs at timber line.
Douglas Fir Climax.— Below the subalpine zone, Douglas fir
(Pseudotsuga taxifolia) is the climax dominant, growing in such
dense stands that subordinate species are negligible. As in the sub-
alpine zone, climax associates differ in the north and south. In the
central and southern Rockies, white fir (Abies concolor) and blue
spruce (Picea pungens) are found in relatively small numbers and
mostly on moist sites. In the north, grand fir (Abies grandis) is an
associate west of the continental divide and principally on west
slopes. East of the divide, Picea glauca of the northern conifer

264 THE STUDY OF PLANT COMMUNITIES ■ Chapter X

FlG. 134. Montane zone climax forest of Douglas fir (Pseudotsuga taxi-
folia) and white fir (Abies concolor) in Colorado.— U. S. Forest Service.

forest shares dominance with Douglas fir and extends southward
through the montane zone as far as the Black Hills.

Dry, exposed ridges in both the montane and subalpine zones
support open stands of pine, including several species characteris-
tic of timber line. P'mus strobiformis is important in the south. P.
aristata occurs in northern Arizona and southern Utah and Colo-

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 265

rado, while P flexilis is more common northward to where P. albi-
caidis takes over in the northern Rockies.

Fire in the Douglas fir climax results in the establishment of
lodgepole pine or aspen stands, which bear the same relationships
here as in the subalpine zone.

Ponderosa Pine Climax.— Below the Douglas fir, is a belt in
which Pinus ponderosa or a close relative forms a relatively open
climax forest that becomes savannah-like with decreasing altitude.
The widely spaced trees form little shade so that the ground cover
is made up of grasses, among which numerous species of Festuca.
Agropyron, Poa, and Muhlenbergia are important. Between the
zone of Douglas fir and the drier, lower altitudes with pure stands
of ponderosa pine is a fairly broad transition where the two trees
may share dominance.

Although the climax is termed ponderosa pine, the species is
dominant only in the northern Rockies to the west of the con-
tinental divide. Elsewhere it is replaced by or in association with
closely related varieties whose ecological characteristics are sim-
ilar. Pinus ponderosa var. scopidorum is the important tree on the
east slope in the north and throughout the zone southward. In the

Fig. 135. Subclimax stand of lodgepole pine in Montana.— U. S. Forest
Service.

266 THE STUDY OF PLANT COMMUNITIES • Chapter X

southern Rockies, the substitutes are P ponder osa var. arizonica,
P leiophylla, and P. latifolia.

The only exceptions to ponderosa pine dominance are found
along streams and drainage lines where narrow-leaved cottonwood

FIG 136 Climax forest of ponderosa pine (Pinus ponderosa) in typical
open stand. Montane zone, Arizona.-L7. S. Forest Service.

(Populus angustifolia), the commonest tree, forms postclimax
stands with P. acuminata and P sargentii in association. Aspen
(P tremuloides), in glades, and box elder may also occur frequent-
ly on these moist sites. Although fires are common, in dry summers,

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 267

favored by the grasses of the forest floor, they are rarely severe
enough to kill the fire-resistant older trees. That pine seedlings are
destroyed is indicated by the even-aged groups of saplings, each
of which can be related to a series of summers that were free of
fire. Severe fires in the upper part of the ponderosa pine zone may
be followed by stands of lodgepole pine. Lumbering and over-
grazing often result in the development of a dense scrub made up
of species from the oak-mountain mahogany zone.

FlG. 137. Characteristic open stand of piiion- juniper, and the transition
from sagebrush desert —U. S. Forest Service.

Vinon-Juniper Climax- This open forest of widely spaced, small
trees (ten to thirty feet) forms the lowest coniferous zone in the
Rockies and, on many of the low ranges of the Great Basin, repre-
sents the only zone present. It is, therefore, typical of the inter-
mountain region as well as forming a distinct zone in the southern
Rockies. Although it is fairly constant in appearance and charac-
teristics over its wide range and extensive acreage, several species
with restricted ranges are involved. The junipers include Juniperus
scopulorwn, J. monospermy J. utahensis, J. occidentalism J. pachy-
phloea, J. mexicana, and others, and the pinons, or nut pines, are
varieties of Finns cembroides (edulis, monophylla, parry ana).

The type extends from northern Mexico along the west slope
of the Rockies to the Snake River in Idaho, beyond which it con-
tinues into southern Alberta with piiion replaced by limber pine.
Along the east slope, its northern limit is in Colorado although it
is represented northward through Wyoming by Juniperus scop-

268 THE STUDY OF PLANT COMMUNITIES • Chapter X

ulorum, often with sagebrush in association. Pinon-juniper is com-
pletely lacking in Sierran zonation, which goes directly from
Artemisia and Furshia to Pinus ponder os a. However, almost with-
out exception, it occurs on every westernmost range and mountain
of the Great Basin, often lying just across a valley from the base
of the Sierra.

■ ■-■•,:.•.-.:.-.•.-. ;v(j(.mv.:j*;K

mmis&m

FlG. 138. An example of the scrub oak-mountain mahogany zone in the
foothills near Colorado Springs, Colo. Quercus gcmibellii predominates here
with Cercocarpus parviflorus and Rosa arkansas as associates. Although the
scrub is sometimes taller, its open, irregular distribution is typical.— P/joto by
R. B. Livingston.

The openings between trees support a grass cover (Bouteloua,
Stipa, Agropyron, Poa) and numerous other herbs, together with
a few shrubs (Ceanothzis, Cercocarpus, Purshia, Coivania, Ar-
temisia, Opuntia) characteristic of the next lower zone. Over-
grazing or fire may result in the temporary dominance of these
shrubs.

Oak-Mountain Mahogany Climax .—The transition from the
conifer forest of the lower slopes to the treeless plains and pla-

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 269

teaus may be marked by a zone of broad-leaved scrub. The zone is
widest and best developed in the southern Rockies, narrows and
becomes discontinuous in the central Rockies, and fades out en-
tirely farther north. The components of the community vary, but
oaks (Quercus gambellii, Q. gimnisoni, Q. undulata, Q. fendleri,
Q. emoryi, and others) are the largest (up to thirty-five feet) and
most conspicuous dominants in the south. North of the latitude
of Denver, Colorado, the oaks are spottily represented, and moun-
tain mahogany (Cercocarpus parviflorus, C. ledif otitis, etc.) is
dominant. Other important associates include Rhus triloba, Pur-
shia tridentata, Fallugia paradoxa, Amelanchier spp., and Sym-
phoricarpos spp., any of which may assume local dominance. The
vegetation does not form a continuous cover but occurs in dense
clumps, or even as individual plants, separated by areas of grass-
land or desert vegetation.

The Black Hills — Although they are now isolated, the Black
Hills are geologically and ecologically related to the Rockies.
They deserve especial mention because of their mixture of eastern,
western, and northern species. Because the altitude is only a little
over seven thousand feet, the montane zone is chiefly represented.
There is no Douglas fir present. Instead, Picea glauca, which ex-
tends southward from Canada along the east slope of the Rockies
as an associate of Douglas fir, here is the only dominant on the high
slopes at the southern limit of its range. Paper birch from the
northern conifer forest is also present. Ponderosa pine dominates
most of the lower slopes, which include most of the area, and
lodgepole and limber pine in small numbers are additional repre-
sentatives from the Rockies. Species from the eastern deciduous
forest are ash, hackberry, elm, birch, and bur oak, of which only
the last attains any size. The scrubby appearance of the commu-
nity, as well as its distribution along the lower margin of the coni-
fer forest, suggests the oak-mahogany zone of the Rockies.85

Sierra Nevada Forest Complex.— The area here considered in-
cludes the southern portion of the Cascade Mountains and the
Sierra Nevada, which together extend from Oregon southward
along the eastern boundary of California as the innermost ranges
of the coastal mountain system. The long west slope of the Sierra
rises gradually to altitudes of 14,000 feet and more, but the east

270 THE STUDY OF PLANT COMMUNITIES ■ Chapter X

slope drops abruptly to the floor of the Great Basin, which lies at
about 4,000 feet. At the base of the west slope, there are only ten
to fifteen inches of rainfall and a long, unbroken, dry summer sea-
son. Upward precipitation increases, temperatures decrease, the
dry summer season shortens, and a larger proportion of precipita-
tion falls as snow.

FlG. 139. Interior of the red fir (Abies magnified) forest that occupies
most of the subalpine zone of the Sierra Nevada.

Because the general north-south axis of the range lies across the
path of the prevailing westerly winds, climatic conditions for the
region as a whole are influenced by them and east slopes are much
drier than west slopes. Winter precipitation makes up 80-85 per-
cent of the total, and at high elevations, most of the moisture falls

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 271

as snow (thirty-five to seventy feet in the subalpine zone). The
greatest total precipitation occurs in the middle slopes, between
5,000 and 7,000 feet, which support the luxuriant mixed coniferous
forest of the montane zone. The subalpine zone coincides with the
altitudes of greatest snowfall, where precipitation equals about
forty to fifty inches a year.

Fig. 140. Lodgepole pine at 8,800 feet in the subalpine zone, Carson Range
of the Sierra Nevada.— Photo by courtesy of the Agricultural Extension Serv-
ice, Univ. of Nevada.

Subalpine Zo?ie.— This zone extends through an altitudinal range
of little more than 1,000 feet, its limits, varying with latitude, be-
ing between 6,500 and 9,500 feet. The climate may be described
as cool, winter-wet, summer-dry, with a short growing season.

Red fir (Abies magnified) is the important climax species, grow-
ing in dense stands and making up 80-90 percent of the forest.189
Of the associated species, none is an important component of the
climax. Although western white pine (Pima monticola) is con-

272 THE STUDY OF PLANT COMMUNITIES * Chapter X

stantly present in small numbers, it is only a minor constituent.
Lodgepole pine (Finns contorta) is often present, especially in wet
meadows, but its role is primarily successional. Mountain hemlock
(Tsnga mertensiana) and white fir (Abies concolor) occur in an
extremely irregular fashion. Of the shrubs, which are few, Kibes
viscosissimum and Symphoricarpos rotimdifolins are the most

FlG. 141. Virgin forest in the Sierran montane zone of California, in this
instance made up of sugar pine (Pinus lambertiana), ponderosa pine (P. pon-
der-osa), and white fir (Abies concolor).— U. S. Forest Service.

abundant and most constantly represented. The herb flora is also
sparse. Constant species are Chrysopsis breweri, Fedicnlaris semi-
bar bata, Gayophytnm ramosissimum, Firola picta and Monardella
odoratissima. The yellow-green lichen (Evernia vidpina) is con-
spicuously present on the trees throughout the zone.

Although the altitudes in the Sierra are often greater than those
of the Rockies, conditions are severe and timber line is lower,
varying from about 7,000 feet in the north to some 10,000 feet in
southern California. The characteristic trees are Finns albicaidis,

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

273

P. flexilis, and P. balfoiiriaiia.2*1 On exposed, bare, rocky slopes,
Juniperus occidentalis is common at timber line and, especially on
the west slope, at much lower altitudes.

The upper margin of the red fir forest does not commonly ex-
tend to timber line but, instead, grades into a relatively narrow

Fig. 142. Giant redwoods (Sequoia gigantea) of the Calaveras grove, Se-
quoia, Calif— U. S. Forest Service.

274 THE STUDY OF PLANT COMMUNITIES * Chapter X

zone of Finns contorta-Tsuga mertensiana dominance. Although
P. contort a is successional to Abies magnified at lower altitudes, it,
with hemlock, has climax characteristics in this zone. This would
suggest three f aciations for the subalpine zone, namely, white bark
pine or timber line f aciation, lodgepole pine-hemlock through the
upper part of the zone, and red fir, which, from the lower margin
upward, occupies the major part of the zone.

Montane Zone —The altitudinal range of this zone lies between
about 2,000-6,000 feet in the Cascades, 4,000-7,000 feet in the cen-
tral Sierra, and 5,000-8,000 feet or more in the south. Five or six
principal species have climax characteristics and may appear in
any combination at any altitude. However, the upper and lower
parts of the zone tend to have consistent vegetational differ-
ences.69' 144 White fir (Abies concolor) is usually the important
dominant in the upper part of the zone, sometimes in pure stands,
and decreases markedly at lower elevations. Lower down, incense
cedar (Libocedrus decurrens), predominating on the most favor-
able sites, sugar pine (P. lambertiana) , Jeffrey pine (P. jeffreyi),
ponderosa pine, and Douglas fir are the species of importance.
Douglas fir is more abundant in the north than in the south.151
Sugar pine and Jeffrey pine are more conspicuous than ponderosa
pine at the upper altitudes, a logical arrangement since the latter
is the most drought-resistant of the major species.

Fire subclimaxes are formed by Pinns attenuata, P. muricata, and
P. radiata in different parts of the range268 although preceded by
dense chaparral communities of species of Arctostaphylos, Ceano-
thus, Rhamnus, etc., which may last for years.

Included in the montane zone, on the western slope, are the for-
ests of giant redwood (Sequoia gigantea), at altitudes of 4,500-
6,000 feet. Once widespread, they now occur only southward
from the latitude of San Francisco in a disrupted zone. Their pres-
ent best development is in the central Sierra where they reproduce
but do not spread. Sugar pine, ponderosa pine, and incense cedar
are common associates.

Foothills (Woodland) Zone —As in the Rockies, the vegetation
of the lower slopes and foothills is made up of coniferous and
scrub associations, but they are not as sharply separated here. The
zone ranges between about 1,500 and 4,000 feet. In the upper part,

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

275

digger pine (P. sabiniana) and blue oak (Q. donglasii) are the dom-
inants, forming typical open, or woodland, stands. The lower alti-
tudes are characteristically covered with close-growing, ever-
green scrub, or chaparral, in which Ceanothns spp. and Arctosta-
phylos spp. predominate. Common associates are several scrub
oaks (Q. rwislize7?i, Q. chrysolepis, Q. dumosa)y Aesculns calif or-

FlG. 143. Characteristic open oak woodland of the Sierran foothills. Se-
quoia National Forest, Calif.— U. S. Forest Service.

nica, Rhamnus calif ornica, and numerous other species are repre-
sented.

East Slope.— Although the same zones are present on both the
west and east slopes, many of the generalizations made above must
be qualified for the east slope because of its less favorable condi-
tions. The red fir forest occurs only in restricted areas on the east
slope, such as in the Carson Range east of Lake Tahoe and locally
in the northern Sierra. The subalpine zone is represented, there-

276 THE STUDY OF PLANT COMMUNITIES • Chapter X

fore, largely by the timber-line pines and patches of lodgepole
pine. The montane and foothill zones extend to high altitudes, and
the vegetation is poorly developed. Finns jeffreyi is the important
species of the montane zone in which the open forest has little
resemblance to that of the west slope. The woodland forest is
practically absent. Although pifion- juniper occurs as a major zone
on the next ranges across the valley, it is not found on the east
slope of the Sierra except where an occasional high spur extends
eastward. The scrub zone is sometimes made up of oak and moun-
tain mahogany as in the Rockies, but is more often represented by
species from the desert below (Artemisia, Purshia, Chrysotham-
nus, etc.), which, especially on areas of disturbance or fire, may be
found high up in the montane zone as well as on the lower slopes.

Pacific Conifer Forest.— This area parallels the coast from north-
ern tree limits in Alaska southward to central California. Coastal
mountain ranges with varying altitudes are included throughout
its length. The climate, tempered by the Pacific Ocean, is mild
and without extremes. Although Alaskan winters are cold, subzero
temperatures are uncommon along the coast. Southward, tempera-
tures are progressively less severe until, in Oregon and California,
frosts are rare. Precipitation is adequate to heavy (30 to 150
inches), and the humidity is always high, producing an extremely
favorable P/E ratio. The southern part is winter-wet with no
snow; here fog compensates for the summer drought. Northward,
the summer dry season shortens until, in Alaska, there is none.
Northward, too, there is an increase in the proportion of precipi-
tation falling as snow. In the higher mountains, it may be entirely
snow with falls as great as sixty to sixty-five feet a year.

The coastal forest is primarily montane in character, although
ranging from sea level to altitudes of 5,000 feet. Only in the United
States, as in the Cascades, and for a short distance into British
Columbia does it include a subalpine forest. Here it is well de-
veloped, but the dominants are derived from the Rockies (Abies
lasiocarpa), the Sierra (Tsnga mertensiana) , as well as the coastal
forest (Abies amabilis, A. nobilis). Northward, the zone becomes
fragmentary or disappears entirely.

Species of the coastal forest are most fully represented in the
general vicinity of Puget Sound, and the best development of the

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 277

forest is indicated by the luxuriant vegetation of the Olympic
Peninsula. Here the ranges of all the major species overlap and
most of the trees attain their maximum size. The climax dominants

FlG. 144. Douglas fir (Pseudotsnga) and western arborvitae (Thuja pli-
cata) in the coastal montane forest. Snoqualmie National Forest, Wash.—
U. S. Forest Service.

are western hemlock (Tsuga heterophylla), arborvitae (Thuja pli-
cata), and grand fir (Abies grandis). Subordinate broad-leaved spe-
cies and many herbaceous species are associated in abundance.133
Douglas fir, which reaches its greatest size here, is the most abun-
dant and widespread species, but it occupies drier sites, is relatively

278 THE STUDY OF PLANT COMMUNITIES • Chapter X

FlG. 145. Pacific coastal forest in California showing redwood (Sequoia
sempervirens) predominating and Douglas fir in association. Conspicuous
subordinate species are Lithocarpus densiflora, Rhododendron californicimi,
Gaidtheria shallon, Vaccinium spp., Polystichum sp.— U. S. Forest Service.

intolerant of shade, and is the major dominant after fire. It is,
therefore, subclimax in nature.124

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

279

To the north of the Puget Sound region, Sitka spruce (Ficea
sitchensis) becomes increasingly important as the forest becomes
more closely associated with coastal conditions. Although it has
subclimax characteristics near its southern limits, Sitka spruce be-
comes, with Tsuga heterophylla and T. mertensiana, an important
climax dominant in the northern extension of the forest.75 At its
extreme limit in Alaska, the coastal and boreal forests merge and
both P. sitchensis and P. glanca are found at timber line advancing
into the tundra.110

Southward, the important species of the Puget Sound center

&«s?

FIG. 146. Successional community of western white pine (Pinus monti-
cola) and western larch (Larix occidentalis) in Idaho. Understory of Thuja
plicata and Tsuga heterophylla —U . S. Forest Service.

extend down the low coastal mountains into Oregon with Port
Orford cedar (Chamaecy parts laivsoniana) as an added climax
species and Douglas fir of relatively greater importance.195 Along
the coast, however, Sitka spruce is replaced by redwood (Sequoia

280 THE STUDY OF PLANT COMMUNITIES ' Chapter X

sempervirens), which, in pure stands, closely follows the limits of
the fog belt71 to below San Francisco and fades out southward.

If the ranges of the principal species of the Puget Sound area
are mapped, they appear in the form of a peninsula extending east-
ward across northern Washington and southern British Columbia
and expanding north and south on the west slope of the Rockies.85
The coastal dominants extending into this area are Tsuga hetero-
phylla, Thuja plicata, and Pseudotsuga, which occupy a zonal posi-
tion between the normal Douglas fir and spruce-fir zones of the
Rockies. Although the importance of hemlock and arborvitae de-
creases eastward and Douglas fir increases, the zone remains dis-
tinctive largely because of the species peculiar to the forests de-
veloping after fire. The two principal successional trees are western
larch (Larix occidejitalis), which is endemic to the peninsula area,
and western white pine (Finns monticola) , which grows more
abundantly here than anywhere else. The presence of Abies
grandis in association with these species indicates the coastal af-
finities.

Daubenmire85 points out that this eastward overflow of coastal
species marks an area in which steady winds blow inland from the
coast, following a well-developed storm track, and thereby extend
the coastal climate far inland. This theory is supported by the
superior development of the coastal species in the peninsula on
westward slopes at intermediate altitudes and their occurrence in
the Rockies only in the storm path and west of the continental
divide.

Broad-Sclerophyll Formation.— As the name indicates, major
species in both associations of this formation have thick, hard,
evergreen leaves. One climax is dominated by trees and termed
broad-sclerophyll forest. The other is a shrub climax called chap-
arral. Both reach their best development on the coastal ranges of
southern California, but their ranges extend from southern Oregon
southward through the coast mountains, as well as through the
Sierra Nevada foothills, into Lower California. Several of the
species are found on the east slopes of the Sierra, and some appear
in the desert woodland zone on the lower slopes of the Rocky
Mountains.

The climate of the sclerophyll region is mild-temperate to sub-

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION

281

tropical with long, dry summers and heavy winter rainfall. Total
precipitation is not less than ten or more than thirty inches, and,
of this amount, no more than 20 percent falls in summer. In this
area, desert vegetation appears where precipitation is less than ten
inches, and, if it is over thirty inches, conifer forest is dominant.72
The two climaxes may be found in alternating patches in almost
any part of their more or less coinciding ranges. However, chap-
arral occupies the greatest area and is climax in the south where it

Fig. 147. Broad sclerophyll forest (Quercus agrifolia, Arubutus, etc.) on
north-facing slope (foreground and right). Chaparral on south-facing slope
(left). Santa Lucia Mountains, Calif.— Photo by W. S. Cooper.

grades into desert, and sclerophyll forest is climax in the north and
at the margin of montane conifer forest where its variations may
be a part of the woodland zone. Where found together, the two
communities bear no successional relationship to each other. The
forest consistently appears on north slopes and the better sites,
chaparral on south slopes and drier sites. The forest is postclimax
in the south, and chaparral is preclimax in the north.

Sclerophyll Forest— The important evergreen forest trees are
Quercus agrifolia, Q. chrysolepis, Q. ivislizeni, Lithocarpus densi-
flora, Umbellularia californica, Arbutus menziesii, Castanopsis
chrysophylla, and Myrica californica. Several deciduous trees are
almost as characteristic, as are a number of shrub and herb associ-
ates. The dominants may occur in various combinations related to
altitude and exposure.

282 THE STUDY OF PLANT COMMUNITIES * Chapter X

Chaparral- -This community extends its dominance over a wide
area and a diversity of habitats, and its composition is proportion-
ately diverse. It includes at least forty species of evergreen shrubs
with varying degrees of dominance and importance, which may
occur in many combinations but which invariably form low, dense

FIG. 148. Chaparral in the Santa Lucia Mountains, Calif. Smooth cover at
top, mostly Adenostoma. Light-colored shrubs in shallow ravine at left, Arc-
tostaphylos glauca. Grades into broad sclerophyll forest in deep ravine at
right.— Photo by W. S. Cooper.

thickets. The most important and constant species is chamiso
(Adenostoma jasciciilatnm). The numerous species of manzanita
(Arctostaphylos) are scarcely less characteristic, and of these A.
tomentosa is the widest ranging. Others with high constancy are
Heteromeles arbutifolia, Ceanothus cimeatus (9 other spp.), Quer-
cus dumosa, and Cercocarpus betulaejormis.

Fires.— The long, dry summers and the nature of sclerophyllous
vegetation make frequent fires the rule. A study in the Santa
Monica mountains showed that chaparral stems were mostly about
twenty-five years of age, and a stand without fire for fifty years
was considered old. An ordinary fire causes chaparral to sprout
profusely, and then, come back to normal within ten years.12 Fire
usually favors the extension of chaparral at the expense of sclero-
phyll forest. Too frequent fires, however, may cause the death of
chaparral and its replacement by grassland. Undoubtedly, the orig-

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 283

inal extent of sclerophyll dominance has been much reduced by
fire, since, once they are destroyed, the return of the sclerophyll
species is long delayed.

Desert Formations.— The major area of the North American
desert extends from southeastern Oregon and southern Idaho
southward through the Great Basin, including most of Nevada
and Utah except high elevations, continues southward into south-
ern California and western Arizona, down most of the peninsula
of Lower California and, on the mainland, through Sonora as far
south as the Yaqui River. The highlands of eastern Arizona and
western New Mexico interrupt the continuity of desert, but from
south-central New Mexico, there is almost continuous desert
through eastern Chihuahua and most of Coahuila in Mexico.238

In spite of the great extent of this area, there are certain environ-
mental features characteristic throughout. Precipitation is low and
erratic; temperatures of air and soil are extremely high by day and
drop abruptly at night; atmospheric humidity is usually low, and
bright sunny days are the rule. These factors serve to explain why
predominating plants are those that can survive desiccation with-
out injury or that store water in their succulent tissues. This is not
to imply that desert vegetation is uniformly similar throughout.
Climatic differences, associated with latitude and altitude, are ac-
companied by differences in species and life forms. Locally, the
physical differences in topography, exposure, and soils produce
distinct vegetational variations just as in moister climates. Finally,
there are numerous undrained depressions into which the water of
winter rains flows and, upon evaporation, deposits the silts and
clays it has transported as well as salts of various kinds. The re-
sulting mud flats (playas) in themselves constitute a special habitat
with associated species, but the nature and concentration of salts
in the soil is even more effective in controlling the communities
there.

Four desert areas are distinguishable on the basis of regional en-
vironments and, likewise, by the nature and importance of the
major dominants :238 namely, the Great Basin, Mojave, Sonoran,
and Chihuahua deserts. In each of these areas there are communi-
ties that occur with minor variations wherever conditions are not
extreme. These may be recognized as climax. Other communities,

284 THE STUDY OF PLANT COMMUNITIES * Chapter X

which seem equally permanent, are found only in special habitats.
Succession, as ordinarily conceived, is almost nonexistent since re-
action of the vegetation is negligible. Unless there is marked dis-
turbance, most communities remain indefinitely unchanged and
dominant in their special habitats. It seems best, therefore, to pre-
sent the characteristics distinguishing the four deserts and to indi-
cate the dominant vegetation in different habitats with less than
the usual emphasis upon climax or its relationships. What follows

,-&:■ .■,■*. . '■ . » ■■■■■>■• -■■■■ ■■'■'■'' v-:V* %■

FIG. 149. Sagebrush desert (Arte?nisia tridentata) northwest of Reno,
Nev.— Photo by W.D. Billings.

is adapted almost entirely from Shreve's238 excellent summary of
desert vegetation except for the distinction made here between
Sagebrush and Desert Scrub Formations.

Sagebrush Formation- Great Basin Desert— There is physio-
graphic, climatologic, and vegetational unity throughout all the
Great Basin area north of southern Nevada and southern Utah.
The wide valley floors, lying at about 4,000 feet, are interrupted
by numerous ridges, often rising to more than 8,000 feet, and de-
pressions of the playa type. The meager rainfall, four to eight
inches, is heaviest in spring but may come in any season. Tempera-

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 285

tures are not as high as farther south, and frosts are common. The
combination of lower temperatures, lower evaporation rates, and
better distribution of rainfall explains the use of the term, "semi-
desert;' for the area. Likewise distinctive is the growth form of the
dominants, made up largely of shrubby chenopods and composites,
which further supports the desirability of its recognition as a for-
mation distinct from the scrub of the southern desert.

-"I

'V-S4^^^^*^-.-^-:i^ />^&.^Zte./.l»f*.A*:~^><-x~*-S,J*f~M.^*«r*X

Fig. 150. Typical dry desert expanse with shadscale (Atriplex conferti-
folia) dominance. Mineral County, Nev. Characteristic gravelly desert pave-
ment shows here .— Photo by W. D. Billings.

The two major communities are simple, with few dominants in
each, and often extend uninterrupted for miles. The sagebrush as-
sociation is dominated by Artemisia tridentata (common sage-
brush), which is climax in the northern portion of the Great Basin
or at relatively high altitudes. The shadscale association, with shad-
scale (Atriplex confertifolia) and bud sage ( Artemisia spines c ens)
as its important species, ranges through the south and at low alti-
tudes. In its northern and eastern distribution, shadscale is found
on heavy lowland soils containing some alkali, but, to the south, it
is climax on gray desert soils with a shallow carbonate layer and
regardless of salts. Sagebrush tends to occur on brown soil, either

286 THE STUDY OF PLANT COMMUNITIES * Chapter X

sandy or clayey, with the carbonate layer at a deeper level and
with a minimum concentration of salts.21

The controlling effect of salts on community structure has been
amply demonstrated for different parts of the area.21' 98> 134 Zonal
patterns around playa lakes are the same everywhere (see Fig. 86).
Where flooding is periodic and salt content excessive, vegetation

~~i

FlG. 151. Creosote bush (Larrea divaricata) with Franseria dumosa in as-
sociation as is typical of much of the Mojave Desert. Numerous desert an-
nuals can be seen.— Photo by W. D. Billings.

is absent or dominated by glasswort (Salic ornia spp.) or iodine
bush (Allenrolfea occidentalis). With somewhat less salt, shadscale
and greasewood (Sarcobatus vermiculatus) or red sage (Kochia
vestita) are dominant. Away from the playas on soils with a mini-
mum of salts, sagebrush may be the major species.

Many other species occur, of course. They are mostly semi-
shrubs with the same growth form. There are numerous species of
Atriplex and Artemisia. Chrysothamnus puberulus, Grayia spin-
osa, Coleogyne ramosissima, Eurotia lanata, Purshia tridentata, and
others are variously associated with the major species or sometimes

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 287

assume dominance under local special conditions. Several species
of Ephedra are characteristic.

Desert Scrub Formation.- Mojave Desert— This, the smallest of
the desert units, lies almost entirely in California below the south-
ern end of the Sierra Nevada. Physiographic conditions are similar

FlG. 152. Joshua tree (Yucca brevifolia), characteristic of the northern
Sonoran desert, particularly in the transition from creosote bush dominance
to shadscale of the sagebrush formation.— Courtesy Univ. of Nevada Agri-
cultural Extension Service.

to the Great Basin but elevations are generally lower (1,000-4,000
feet). The irregular precipitation of less than five inches is dis-
tributed over fall, winter, and spring. Summers are very hot and
dry. The area includes Death Valley, with a minimum elevation of
480 feet below sea level. Its infrequent maximum rainfall is two

288 THE STUDY OF PLANT COMMUNITIES * Chapter X

inches, and official records show at least one period when tempera-
tures held above 100° E for 538 days.192

Conditions are not too different from those of the Great Basin
although somewhat more extreme. This is borne out by the vege-
tation, which includes many of the same species, their distribution
controlled here, too, by soil texture and salt concentration. Certain
character species do stand out, however, and this justifies the vege-
tational distinction from the Great Basin. At the upper elevations
(3,000-4,000 feet) and in the transition from sagebrush, with maxi-
mum precipitation (near five inches), Joshua tree (Yucca brevi-
folia) is conspicuous. With decreasing altitude and precipitation,
creosote bush (Larrea divaricata), with burro weed (Franseria
dumosa) in association, becomes the major dominant. This com-
munity occupies 70 percent of the total area of the iMojave Desert.

Sonoran Desert.— The lowlands around the Gulf of California
in Mexico and Lower California, which lie chiefly below 2,000
feet, constitute the Sonoran Desert. Much of the area is made up
of dunes and sand plains. Precipitation is extremely uncertain, not
exceeding two to four inches in the vicinity of the Gulf, although
increasing some with altitude. Its effectiveness is counteracted by
the extremely high temperatures.237

Fig. 153. Sahuaro (Carnegiea gigantea), the giant of the columnar cacti
that characterize the uplands of the Sonoran Desert.— U. S. Forest Service.

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 289

The low plains are dominated by Larrea-Franseria, with various
associates, as in the Mojave Desert. Because drainage here is not
internal, margins of streambeds support a distinctive mixed com-

FlG. 154. Mesquite (Prosopis chilensis), a common ground-water indica-
tor in the desert scrub formation —Courtesy Univ. of Nevada Agricultural
Extension Service.

munity including species of Prosopis, Cercidium, Olneya, Dalea,
etc. In the higher elevations of Arizona and northern Sonora
(1,000-2,000 feet), there is a great mixture of species and life
forms. Although numerous species characteristic of the other des-
erts are present, Cercidium microphyllum is a dominant with num-
erous arborescent and columnar cacti, including Carnegiea giga?i-
tea, Lemaireocereus schottii, and many species of Opuntia. The
variable topography of the peninsula of Lower California supports
an equally variable flora including many species. Near the coast,
there are more leaf succulents than in any of the other desert
areas.

Chihuahua Desert— Extending from southern New Mexico
southeastward to western Texas and down into Mexico, much of
this area is interrupted by high mountains and lies between 4,000

290 THE STUDY OF PLANT COMMUNITIES • Chapter X

and 6,000 feet. Precipitation varies with altitude from three to
twelve inches and falls largely in summer. Temperatures are some-
what lower than in the Sonoran Desert, and frosts are not un-
common.

Under these conditions, the communities are not as complex as
those of the Sonoran Desert or as simple as those of the Great
Basin, but there is much regional variation. Shrubs and semishrubs
predominate with a great variety of inconspicuous stem succulents
in association. Ocotillo (Fonquieria splendens), which is found
throughout the area, creosote bush (Larrea tridentata), and mes-
quite (Prosopis juliflora) are the only three species common in
the Sonoran Desert that are also important and widespread here.
A number of species are conspicuous because of size or unusual
form. Yucca, Nolina, and Dasylirion are large semisucculents.
Agave and Hechtia are particularly abundant leaf succulents.
Leafless, green-stemmed trees, columnar cacti, and Dasylirion
longissimwn with its six-foot, linear leaves, are examples of locally
important species of striking appearance.

Grassland Formation.— Grasses are climax dominants over all
the vast area extending from southern Saskatchewan and Alberta
to eastern Texas, and from Indiana and the western margin of
deciduous forest westward to the woodland zone of the Rockies.
Separated from this major area are the Palouse region of Washing-
ton and the grasslands of the great valley of California. The for-
mation has the greatest extent of any in North America and con-
sequently grows under a great diversity of conditions. This is
possible because of the growth form of the species, their long pe-
riod of dormancy, and the fact that their moisture requirements
are critical only in spring and early summer.

The eastern transition to forest is marked by an annual precipi-
tation of thirty to forty inches from Texas to Indiana and twenty
or twenty-five inches farther north. A high proportion of this
precipitation falls as spring rain, but westward, as the total de-
creases to about ten inches near the Rockies, the proportion fall-
ing in spring and summer also decreases. Temperatures are equally
variable. In the north, the growing season is cool and short, and
subzero temperatures occur for long periods in winter. In the
southern part of the range, frosts may be almost unknown, and

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 291

extremely high summer temperatures are characteristic. Through-
out the formation, late summer dry spells with high temperatures
and drying winds are the rule, but, if there is sufficient moisture
for the grasses during the spring growing period and summer ma-
turation, such extremes affect them but little because of their long
period of dormancy. The hot season with limited precipitation is
probably of great importance in maintaining grassland climax
against the advance of forest.

The increasingly severe moisture conditions from east to west
are accompanied by changes in the dominant species whose com-
binations are distinguishable as associations of the formation. Three
major regions are recognizable either by climate or vegetation, or
both. Their limits, climate, and vegetation have been summarized
and the important regional and local studies of grassland have been
listed in a concise presentation by the late Dr. J. R. Carpenter.52
This condensation of grassland information could well be used as
the starting point for any consideration of the nature and distribu-
tion of grassland. The great number of classifications attempted
for grassland communities and the disagreements as to major dom-
inants and most important species implied by the terminology sug-
gest the complexity of the formation. Probably, too, there is a
suggestion of much more variation regionally than might at first
be supposed. Of necessity, we are restricted here to a simple pres-
entation. On this basis, the discussion will deal with only three
major associations, which may be termed Tall Grass Prairie, Mixed
Prairie, and Short Grass Plains. Some authorities recognize as many
as seven associations,57 and, even then, most of these can be di-
vided into several faciations. Furthermore, a detailed discussion
must recognize within each faciation the usually distinct upland,
slope, and lowland variations.

Tall Grass Prairie— Sometimes called "true prairie" this associa-
tion borders the deciduous forest, receives the most rainfall, has
the greatest north-south diversity and the greatest number of
major dominants of the association. Bunch grasses are the con-
spicuous species, for many of them grow in excess of six feet tall,
but sod-forming species are also dominants. Because of the gen-
erally favorable climatic and soil conditions, most of the area is
cultivated and little of the original vegetation remains today.

292 THE STUDY OF PLANT COMMUNITIES • Chapter X

FlG. 155. Tall grass, or true prairie, community in which Andropogon
scoparius, Bouteloua gracilis, and Sporobolus heterolepis are the most impor-
tant species.— Photo by R. B. Livingston.

The long list of major dominants includes tall grasses, such as
Stipa spar tea, Andropogo?i furcatus, and Sorghastrum nutans;
medium grasses, such as Andropogon scoparius and Bouteloua
curtipendula; and the short grasses, Bouteloua gracilis and B. hir-
suta. The association of dominants with topography should be in-
dicated at some point even though it is impossible to recognize it
throughout our discussion. The following groupings are not un-
common for Tall Grass Prairie.

UPLANDS

Agropyron repens
Bouteloua gracilis
B. curtipendula
Andropogoji scoparius
Poa pratensis
Sorghastrum nutans

SLOPES

LOWLANDS

Poa pratensis
Sorghastrum nutans
Koeleria cristata
Andropogon furcatus
Stipa spartea

Poa pratensis
Sorghastrum nutans

Andropogon furcatus

Agrostis alba
Spartina pectinata
Panicum virgatum

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 293

This distribution is not the same everywhere. In the north,
Koeleria and Stipa appear in the uplands, and Poa and Sorghastrum
do not appear at all. In the central region, Panicimi virgatum, Birt-
bilis dactyloides, and Boutelona hirsnta are added to the uplands
and Sporobolus heterolepis and S. cryptandrus to slopes. The

Fig. 156. Mixed grass prairie in which Bouteloua gracilis, Stipa co?nata,
and Calamovilja longifolia are the principal species. Colorado.— Photo by R.
B. Livingston.

southern faciation, sometimes regarded as a separate association, is
even more distinct, especially because of added species in the up-
lands, such as Stipa leucotricha, Andropogon saccharoides, A.
tener, and A. ternarius.

There has been much discussion and study of the eastern mar-
gin of Tall Grass Prairie, its extension as a "peninsula" into Illinois
and Indiana, and the isolated areas farther east, particularly in

294 THE STUDY OF PLANT COMMUNITIES * Chapter X

Ohio. Because here it includes somewhat different combinations
of species, some of which are dominant, the community is re-
garded by some as a separate association. The predominating tall
grasses, as well as other basic similarities, make it reasonable to

FIG. 157. Mixed grass community in Arizona in which grama grasses pre-
dominate.— U. S. Forest Service.

others to consider the prairie peninsula as a f aciation of Tali Grass
Prairie to which it bears a postclimax relationship. The soils with-
in the peninsula are prairie soils although the climate is now that
of forest climax. The community may, therefore, be regarded as
preclimax to the forest, maintained by edaphic conditions.

Mixed Grass Prairie.— Although the mixed grasses occupy an
area between that of the tall grasses and short grasses and the dom-

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 295

inants are derived from both these communities, it is generally
agreed that there is sufficient unity and distinctness to justify as-
sociational rank. Important dominants throughout the area are
Bouteloua gracilis, B. hirsuta, Andropogon scoparius, and, except
in the north, Bulbilis dacty hides. In the north, Koeleria cristata,
Stipa spartea, and 5. comata are added dominants, which suggest
the recognition of a northern faciation. Other important species
included among the dominants are Andropogon furcatus, Sporo-
bolus cryptandrus, and several species of Stipa.

FlG. 158. Short grass plains pastured to sheep in Wyoming.— Photo by W.
D. Billings.

The western limit of the association may be taken as the line
where tall grasses disappear and beyond which only short grasses
are dominant. Since the tall grasses require available soil moisture
to a depth of twenty-four or more inches during their active
growing season, the limit of mixed grass prairie is a line beyond
which precipitation is insufficient to provide moisture to this
depth. The eastern limit is not as sharply defined but is also de-
termined by soil moisture, since mixed prairie is marked by prairie
grasses in bunch-grass habit sharing dominance with permanently
established short grasses.3 Thus the area forms a strip from Sas-
katchewan through the central Dakotas, Nebraska, Kansas, and

296 THE STUDY OF PLANT COMMUNITIES * Chapter X

western Oklahoma into Texas. The sand hills of Nebraska are an
exception, for here soil conditions are such that postclimax tall
grasses predominate. During protracted dry periods, the short
grasses increase at the expense of the moisture-requiring tall
grasses.267 Thus the boundaries of the association are not particu-
larly static and are represented by a wide transition zone.

FlG. 159. Short grass range in Colorado under average grazing and con-
sequently in good condition.— U. S. Forest Service.

Short Grass Plains— Westward from the Mixed Grass Prairie to
the woodland zone of the Rockies, the xeric short grasses are dom-
inant. On the basis of exclosure studies and other observations, the
climax nature of short grasses has been questioned, and the com-
munity has been described as disclimax resulting from overgrazing
an area that would otherwise support mixed prairie.268 This inter-
pretation is gradually gaining favor. Regardless of terminology,
the short grasses are, at present, dominant over the entire area.

The most important species are Bouteloua gracilis and Balbilis

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 297

dactyloides, except north of the Dakotas, where the latter is ab-
sent. Several faciations are recognizable that result from combina-
tions of the major dominants with Stipa comata, or Agropyron
smithii, or Aristida longiseta.

The desert plains area extending from western Texas across
northern Mexico and southern New Mexico and Arizona supports
short grasses, which, although including different species, are re-
lated to short grass dominance. Several species of Bonteloiia and
Aristida predominate. Overgrazing has greatly increased the num-
bers of desert shrubs here, and these include Larrea, Gpuntia,
Flourensia, and several others of which widely spaced individuals
occur everywhere.

Other Grassland Climax— There is evidence that the great valley
of California was once dominated by grasses, which, because of
fire and grazing, have been eliminated except for relict areas. The
latter suggest that the dominants were bunch grasses, which pro-
duced grassland similar in appearance to mixed prairie. Through-
out most of the area it appears that Stipa pulchra was the principal
species, except near the coast. Today introduced annual grasses
occupy most of the remaining grassland areas, especially species
of Avena, Bromus, Festuca, and Hordeum.

The rolling hills of the Palouse region, as well as most of eastern
Washington and Oregon and eastward into Idaho, supported
prairie grasses before being cultivated for wheat production. Al-
though numerous species characteristic of other grassland areas are
present here, the major dominants are distinctive, including Agro-
pyron spicatum, Festuca idahoensis, and Elyimis condensatus. Pos-
sibly much of the sagebrush dominance in this region is only the
result of grazing, and certainly the dominance of the annual,
Br omits tectorum, results from fire and grazing as it does south-
ward in the Great Basin.

The Palouse and California grasslands, in contrast with the major
areas, are products of winter, rather than spring and summer,
precipitation.

Aspect Dominance.— Probably no other formation has such
marked variations in appearance through the growing season.
Since not all the grasses mature at once, there are times when
simple observation might lead to incorrect conclusions as to their

298 THE STUDY OF PLANT COMMUNITIES • Chapter X

relative importance. Associated species other than grasses, often
called forbs, may be seasonally so conspicuous as to obscure the
grasses and, temporarily at least, to give the appearance of dom-
inance.

Tropical Formations.— The truly tropical vegetation of North
America, which occurs only in southern Mexico and Central
America, probably includes as great a diversity of communities as
is usually found in temperate climates. The major controlling fac-
tor in this diversity is moisture, as affected by topography, expos-
ure, and seasonal distribution. Although numerous local studies of
the vegetation of American tropics have been made, it is only re-
cently that a comprehensive classification of the plant communities
has been attempted in the light of modern concepts.15

A misconceived but popular idea of tropical vegetation is un-
doubtedly one which can best be placed in the category of rain-
forest-in-its- jungle-form. But such tangled masses of vegetation
are found only on areas of disturbance and "True rain forest al-
ways gives the impression of the vault of cathedral aisles!'15 It is
made up of many species of tall broad-leaved, evergreen trees in
several strata with the tallest sometimes rising ninety feet to the
lowest branch. Undergrowth is sparse, lianas are few, and epiphy-
tes are not abundant near the ground. Apparently, after disturb-
ance of any kind, such forests are replaced by a tangled jungle of
growth that is almost impenetrable. The rain forest is not wide-
spread because conditions for its development are by no means
everywhere available. It occurs where temperatures are fairly con-
stantly high, precipitation is plentiful (over two hundred inches in
some areas), and on good sites with proper drainage but with a
continuous supply of available water.

It should be re-emphasized that not all tropical vegetation is
rain forest, and to this should be added that not all broad-leaved
evergreen forest is rain forest. The presence in the tropics of
mountains of sufficient height to have permanent snow on their
peaks insures altitudinal zonation similar to that of temperate re-
gions. These mountains may interrupt moisture-bearing winds and
so maintain desert conditions. Seasonal deciduous forests, pine for-
ests, and even tundra are to be found on their slopes. The major
variations in American tropical vegetation have been grouped into

CLIMAX COMMUNITIES : PRESENT DISTRIBUTION 299

six formations, each of which may be divided into from two to
nine associations.15

1. Rain Forest (Optimum formation)

2. Seasonal Formations

3 . Dry Evergreen Formation

4. Montane Formation

5. Swamp Formation

6. Marsh or Seasonal Swamp Formation

The subtropical climate of the southern tip of Florida and the
Gulf coast down into Mexico permits the growth of numerous
species with tropical characteristics and affinities. The palms, the
many broad-leaved evergreens, the mangroves, the many epiphytes
and lianas, and the sometimes jungle-like masses of vegetation are
all suggestive of tropical conditions.

GENERAL REFERENCES

E. LUCY Braun. The Undifferentiated Deciduous Forest Climax and the

Association Segregate.
J. R. Carpenter. The Grassland Biome.
E E. CLEMENTS. Plant Indicators : The Relation of Plant Communities to ■

Processes and Practice.
R. E DAUBENMIRE. Vegetational Zonation in the Rocky Mountains.
J. W HARSHBERGER. Phy to geographic Survey of North A?nerica.
B. E. Livingston and E Shreve. The Distribution of Vegetation in the

United States, As Related to Climatic Conditions.
H. L. SHANTZ and R. ZON. The Physical Basis of Agriculture : Natural

Vegetation, in Atlas of American Agriculture.
V E. SHELFORD (ed.). Naturalises Guide to the Americas.
E SHREVE. A Map of the Vegetation of the United States.
J. E. Weaver and F. E. Clements. Plant Ecology.

CHAPTER XI

THE DISTRIBUTION OF CLIMAX COMMUNITIES
SHIFTS OF CLIMAXES WITH TIME

The present distribution and limits of climax communities are
not necessarily static, nor have they been in the past. Looked at in
terms of geological time, changes of climate must be recognized
that were so extreme that vegetation must likewise have changed
radically. Within relatively recent geological time, glaciation of
northern North America obviously must have produced such
changes in climate that disruption of then existing lines of vegeta-
tional distribution were inevitable. Advance of the ice southward
resulted in constriction of vegetational zones and retreat of species
and growth forms as the climate changed. With the recession of
the ice, there was again a northward advance of species and a re-
adjustment of plant communities as the glaciated area was reoc-
cupied by vegetation. Probably there were several minor advances
and retreats of vegetation correlated with the shifting ice fronts
and the similarly varying climate.

Within historical time, there have been major shifts of climate
producing conditions that may have had serious effects on vege-
tation. There is evidence that early Norsemen who colonized
Greenland were able to carry on a primitive sort of agriculture on

lands alone the southern coast. Between the twelfth and the four-
th

teenth centuries the climate there deteriorated rapidly so that
summers became shorter and colder, the soil remained frozen, and
the colonists disappeared. Today, as for some time past, the reced-
ing glaciers in Greenland indicate an increasingly favorable cli-
mate. Receding glaciers in Alaska have been similarly interpreted.76
In recent years, conifer forest has been advancing into the tundra
in Alaska.110 Periodically, prairie vegetation is invaded for some
distance by forest, and although drought often eliminates such ad-
vances, they may be permanent or, at least, appear so.

That climates have changed over long periods of time cannot
be questioned, and that slow change continues today in certain

300

CLIMAX COMMUNITIES : SHIFTS WITH TIME 301

areas is undoubtedly true. With climatic change, vegetational
change is to be expected. Some modern changes are easily recog-
nized, as indicated above. In highly populated areas, the changes
may be much less obvious because natural vegetation has been
disturbed by man.

PALEO-ECOLOGY

This phase of ecology deals with the history of vegetation, es-
pecially the reconstruction of past climaxes and climates, their
rise, decline, and migration over long periods of geological time.
Its basic source materials are derived from paleontology and geol-
ogy and must be interpreted in terms of what is known of the
ecology of modern organisms.

Tracing changes in modern climax vegetation is a complex proc-
ess involving the use of every kind of evidence available. E B.
Sears' 218 reconstruction of the natural vegetation of Ohio and its
prehistoric development221 illustrates how historical records and
pollen statistics may contribute evidence. A. M. Raup's203 study of
New England climate and vegetation utilizes still other sources of
evidence. Archaeology, zoology, botany, and geology all were
drawn upon in a variety of ways before he concluded that New
England had had a warmer climate within recent years— probably
no more than a thousand years— and that the trend has since been
to the cooler and moister, with parallel vegetational changes.

Knowing that climates have changed, one may be equally cer-
tain that vegetation has varied accordingly. Major alterations in
vegetation may likewise be assumed to indicate modification of
climate. In some instances, however, such shifts have been inter-
preted as purely successional in nature, a point not to be ignored
since succession has gone on in the past as it does today. Change
within historical time, if still in progress, may be observed, or may
become apparent from detailed quantitative and qualitative studies
of transition areas.39 A less reliable source of information is the
historical literature not always dependable, unfortunately, because
of the limited knowledge of the early writers. It is, nevertheless,
a source from which much of value can be learned,36' 203 particu-
larly when the information is drawn from several sources and is
correlated with other kinds of evidence.

The difficulties of reconstructing the vegetational picture dur-

302 THE STUDY OF PLANT COMMUNITIES ' Chapter XI

ing early historical time are as nothing compared with those in-
volved in determining prehistoric climaxes.50 Fossils, variously pre-
served, are the chief source of our knowledge of ancient floras,
many of which have disappeared completely. Considering that
different species and even parts of the same plant are unequally
preserved, it is surprising that we know as much of these old

FlG. 160. Interglacial forest relicts on beach below high tide, Glacier Bay,
Alaska. These hemlock stumps, probably several thousand years old, repre-
sent forest that lived before the last major advance of ice, which buried them
under glacial debris (above beach). Tide action has again exposed the stumps.
—Photo by W. S. Cooper.1

floras as we do. Certainly we know that there have been extreme
climatic changes on various parts of the earth and that with them
have come modifications in vegetation, which sometimes elimin-
ated entire floras.

More recent vegetational history has been given greater atten-
tion because of its direct relationships to our modern flora and,
possibly, because it offers greater probability of solution. Post-
glacial climate and vegetation have been studied more intensively,
therefore, than those of preglacial time. Plant remains, buried and
preserved between layers of glacial drift, have yielded much in-
formation on the amount of time involved, the climate, and the

CLIMAX COMMUNITIES : SHIFTS WITH TIME

303

vegetation. These deposits, often preserved in a natural state as
wood, leaves, fruits, or seeds, have been uncovered by erosion,
excavation, and even in driving wells at considerable depth. Such
findings have been fortuitous largely, since the deposits do not
occur generally and, when stumbled upon, must be brought to the
attention of those interested if they are to be of any scientific

FlG. 161. Well-preserved Pleistocene plant remains found in silt or peat
layers buried under 10 to 12 feet of undisturbed moraine in Minneapolis,
Minn. (1) Collier gon giganteum, (2) Neocalliergon integrijolium, (3) Picea
sp., wood structure almost perfect, (4) Picea sp., wood structure distorted by
pressure, (5) cone of Picea glanca, (6) cone of Picea mariana, (7) cone of
Larix laricina—From Cooper and Foot.11

value. As a result, the information they have yielded is fragmentary
and discontinuous both in time and space.

A more promising approach to the problem began with the
study of the nature and composition of the strata of plant remains
and other sediments that have accumulated in lakes and ponds as
peat or related material.82 These strata may give almost continuous
records back to glacial time, and, since deposits are distributed
over wide areas, their study makes possible the correlation of find-

304 THE STUDY OF PLANT COMMUNITIES * Chapter XI

ings, particularly regarding climate, in one place with those in an-
other. Obviously such studies can not be entirely satisfactory since
they indicate vegetation only within the bogs themselves, or at
their immediate margins, and bog vegetation is not of the climax
type.

POLLEN ANALYSIS

When, in 1916, von Post presented the results of his studies ot
pollen preserved in Swedish peat deposits, an entirely new ap-
proach to the reconstruction of prehistoric vegetation was begun.97

EBERBACH ,

" MM— ■

FlG. 162. A type of sampler frequently used for pollen studies of peat and
marl deposits. It consists of a jacketed plunger that completely closes the
sharpened end of the jacket. After it is pushed down to sampling depth,
using the four-foot extension rods, it is drawn upward a few inches. This
partly withdraws and locks the plunger in the upper part of the jacket.
Then, when forced downward, the jacket cuts a ten-inch sample core-
Courtesy of Eberbach and Sons Company.

Wind-borne pollen is deposited everywhere and much of that
which falls in a lake is preserved in its sediments because of the
low rate of oxidation. Since the pollen of most dominant trees is
wind-borne, the pollen deposited at any one time should include
that of the important tree species in the general vicinity and the
numbers of grains of a species should be indicative of the relative
importance of that species in the surrounding forest at the time.
Because pollen grains of a species are constant in size and form,
genera, and sometimes species, can be identified positively. Conse-

CLIMAX COMMUNITIES : SHIFTS WITH TIME

305

LJ <J

7 1

I
.1

r
i-

CI ,

13-

?! O"

uj o

a *> < w i- i

~i —

D
O

3
O

D r

u -

CD a

3 i

< 2

2 »■

< -

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a. *

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GO
<

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l l l i i i — rr

SO 0

r- i i i i — i i i i — i i i i — i i i i — rrr

A I

FlG. 163. An example of a common form of pollen diagram, which also
illustrates what we know of the vegetational history of the northeastern
United States although derived from one place (Upper Linsley Pond, North
Branford, Conn.) Zone A indicates a spruce-fir forest; the high values for
pine are attributed to over-representation resulting from its light weight and
its abundant production. In the northeast, a secondary maximum for spruce
(A-2) is not uncommon in this period and is thought to represent a local
readvance of retreating ice. Zone B, a warm dry period, shows a pine maxi-
mum and the beginning of warmth-loving, deciduous trees. Then followed
deciduous dominance over a long period, in which hemlock-oak were first
important (C-l), then oak-hickory (C-2), and, with cooler moister condi-
tions (C-3), an increase of chestnut, followed by a reappearance of spruce in
some localities.— Fr om Deevey*9

quently, if samples of lake deposits are taken from the bottom up-
ward to the present surface of the sediment, the pollen content of
successive strata should indicate the nature of the forest, as to
genera and their relative abundance, throughout the period of ac-
cumulation.

Sediments on lake bottoms as well as peat deposits have been

306 THE STUDY OF PLANT COMMUNITIES " Chapter XI

studied. Samples must be taken with care to prevent contamina-
tion, and several types of augers have been devised for the pur-
pose. With these, cores can be cut that, placed end to end, form a
continuous column of material for the entire depth of the deposit.
Borings are made in summer under most conditions, but, since it
is desirable to have them from the deepest part of the depression,
it is often advantageous to make them in winter from the frozen
surface.

Identification and counting of the pollen grains must be done
under a microscope. This necessitates treatment of the samples
with one of the several methods recommended48 to eliminate for-
eign material and to concentrate the grains. Identification is facili-
tated by reference to illustrations274 and by comparison with a
series of grains taken from modern plants. What constitutes an

NORTHERN MtNNESOTA
AND WISCONSIN

NORTH-CENTRAL
STATES

NORTHEASTERN
OCEANIC

FlG. 164. Schematic pollen profiles that show the general picture of what
is known of vegetational history for the eastern United States. F— fir, G—
grassland complex, H— hardwoods except oak, O— oak, P— pine, S— spruce.
Depth shown vertically, percentages horizontally. Although there are differ-
ences relatable to continental and maritime climates, there is regional similar-
ity in the indications of a middle warm, drier period, and the suggestion of
subsequent cooler, moister conditions leading into the present, as well as the
shift toward early proportions of species in the upper portions of the dia-
grams. Succession may be a factor in these latter shifts.— After Sears.2

219

CLIMAX COMMUNITIES : SHIFTS WITH TIME

307

adequate sample in the count of grains is not agreed upon by all
investigators but fewer than 150-250 grains are rarely counted.

When the proportions for genera are known for each stratum,
they are represented in a standardized form, known as a pollen
diagram, in which pollen spectra— the relative importance of each
genus in a stratum— are plotted on horizontal lines, one spectrum
above another to show the progressive changes for genera, which,
are shown on vertical lines. A pollen diagram is no more than a
means of visualizing the pollen spectrum of a section— a vertical
series of samples from the bottom to the surface of a deposit.
Changes in the spectra from the bottom upward are, of course, to
be correlated with time.

The shortcomings and pitfalls of pollen analysis as a method of
determining past vegetation and climate are appreciated by all
who have used it.48' 97 There are sources of error in methods, in
records, which may be incomplete, and in identifications which
may not always be correct, and interpretations may be based upon
inadequate data. Because of its simpler flora and greater amount
of study, the pollen spectrum for Europe is better established and

Age in
years

2O0O

400O

6000

10,000

12000

14.000

16.000

Period itr
Cooler

Moijler

Per/od n'i
Maximum

Warmth
and

Dryness

Period li

Increasing

Warmth

and
Dryness

Period /
Cool-Moisf

Pmus
monhcola

Pjeudolsupa
la*, i folia

Tsuga
hererophy/la

% eo

-hO

4-0

FlG. 165. A composite of ten pollen profiles from the Puget Sound region,
which is indicative of postglacial climate and vegetation in the northwest
although not typical of all areas as to species. The volcanic ash level, present
in all northwestern profiles, is considered to be of common age. Such com-
posite profiles, because they eliminate the sharp fluctuations from level to
level found in individual profiles, give a better picture of the trend of post-
glacial vegetation.— From Hansen.

115

308 THE STUDY OF PLANT COMMUNITIES * Chapter XI

accepted than in North America. Most of our studies have been
made in areas where bogs are common and within a reasonable
range of accessibility of an individual or his students : the north-
west, the north-central states, and the northeast. There is still
much to be done within the glacial area to complete the picture.

It is somewhat surprising that investigators are in as close agree-
ment as they are. Most generally accepted is a postglacial climatic
series beginning with increasing warmth, followed by a period of
maximum warmth and drought, followed by a period of decreas-
ing warmth, the present.224 This is applicable to both Europe and
America. Some students would subdivide these three major pe-
riods, claiming that greater refinement is possible. Others contend
that their data contain no evidence of a warm dry period in North
America.

More studies are certainly necessary in North America before
agreement can be reached as to all phases of the basic normal pollen
spectrum and its meaning in terms of climate. Several scattered
studies have been made of deposits beyond the limits of glaciation,
and these offer real possibilities. Likewise, there must be more ef-
forts to correlate all sources of contributing evidence,89' 203 a truly
paleo-ecological approach : floristic, vegetational, zoological, geo-
logical, archeological, as well as evidence from pollen analysis.

DENDROCHRONOLOGY

Another bioclimatic approach to past history was originated by
an astronomer. Dr. A. E. Douglass, when he began studies of an-
nual growth rings of trees in an attempt to correlate their differ-
ences with climatic variations, presumably related to solar activity.
Cross-dating, or matching the growth patterns year by year, for
modern trees in Arizona was first accomplished in 1904, but its
significance was not fully appreciated until several years later.91
Then a chronology was established from modern times back to
A.D. 1400 by matching ring records of modern trees to the ex-
terior ring records of earlier trees and so on with trees that grew
still earlier. When these records were matched with rings in beams
taken from ancient pueblos, the records became complete back
to A.D. 1299, then to A.D. 700 and, more recently, successively to
A.D. 643, A.D. 500, and finally to A.D. 11. Recent finds suggest

CLIMAX COMMUNITIES : SHIFTS WITH TIME

309

that the chronology will be carried even further back.119 Some of
the record was completed and some of the cross-matching was
made possible by fragments of wood from ancient pueblos and
some even with charcoal, which was better preserved than wood.
It should be noted that an even longer chronology has been worked
out for the giant redwoods, which is complete for 3,000 years.

When the pueblo dendrochronology was completed, it was a
major contribution to archaeology since some thirty prehistoric
ruins were immediately given absolute dates, and later hundreds
more were dated. This usefulness of the method was immediately
apparent to archaeologists, who accepted it and adapted it to their
purposes. At the same time, their findings in archaeology have
contributed to the establishment of dendrochronology as a means
of studying past climate.

Recent ring studies in moist cool regions indicate that no better
climate than the arid Southwest could have been selected for the
initial investigations. In extremely dry regions, growth and size of
rings are closely related to annual precipitation and the correlation
is not complicated by light or temperature effects. It is now known
that in the north, or at high altitudes, tree growth is most respon-
sive to temperature and that in temperate regions with adequate
rainfall, both temperature and moisture factors are reflected in
the rings.90

This does not mean that tree-ring studies are successful only in
arid regions but rather that their interpretation may be more dif-

TO PREHISTORIC
TIMES

THIS YEAR'S

TREE CUT
THIS YEAR

SECTIONS FROM HISTORIC BEAMS

FlG. 166. A diagram illustrating how the bridge method is used to extend
knowledge of dated rings, an important part of the building of complete and
continuous chronologies. The usual desirable overlap is fifty years.— After
Glock.107

310 THE STUDY OF PLANT COMMUNITIES * Chapter XI

ficult elsewhere. That cross-dating and correlation with climatic
variation is possible in moist-temperate climates was demonstrated
by Douglass' studies in Europe and several parts of the United
States. In the Mississippi drainage area, the deviation from the
normal annual precipitation has been shown to affect ring growth
more than total precipitation, but the relationship is modified by
temperature and wind as they influence evaporation.119 Ring
growth in New England has been shown, in a chronology from
hemlock, to have close correlation with climate as indicated by
exceptional and poor crop years.164

Since tree-ring analysis was originally begun with the hope that
it would show solar-terrestrial relationships, it was natural that,
with the establishment of long, dated chronologies, the data should
be studied for cyclic characteristics. Permanent periods, or those
of fixed length, showed no correlation; therefore, this idea was dis-
carded for one of cycle complexes in which any obvious or sig-
nificant recurrence of variation in data was considered to be cyclic.
On this basis, definite relationships were demonstrable between
sunspot activity in the past and terrestrial climate as recorded in
certain long-time chronologies of tree rings. An eleven- (ten to
twelve) year cycle is especially pronounced throughout the old
records and continues to be borne out, in a general way, for mod-
ern conditions. During periods of sunspot maximum, drought is
characteristic, and sunspot minimum is associated with excessive
precipitation. Application of the method to climatic prediction
may be possible as more long-time meteorological records are an-
alyzed and as more tree records of great length are worked out to
show the nature of prehistoric climates on many parts of the earth.

THE RELICT METHOD

As for several other phases of dynamic ecology, we are indebted
to Dr. E E. Clements for recognizing the potentialities of the relict
method, for demonstrating its usefulness, and for a clear and com-
plete exposition of the entire subject. The brief discussion that
follows can hardly avoid being a condensation of his ideas.58

"In the ecological sense, a relict is a community or fragment of
one that has survived some important change, often to become in
appearance an integral part of the existing vegetation!' The con-

CLIMAX COMMUNITIES : SHIFTS WITH TIME

311

cept may be applied to individuals or a species, but is more often
used for communities. It may be used to describe delayed or lag-
ging stages of succession, but it has far greater usefulness in con-
nection with climax vegetation.

The usefulness of relicts lies in their indicator value of past con-
ditions of habitat and vegetation as well as of the causes underly-
ing changes that have occurred elsewhere in the area. A relict

FlG. 167. Relict (postclimax) black spruce forest in a Minnesota bog.—
U. S. Forest Service.

community having remained relatively unchanged because of pe-
culiar local conditions is an actual sample of, or shows strong
similarities to, previous vegetation. At the same time, the peculiari-
ties of the relict habitat are indicative of environmental conditions
previously characteristic of the area as a whole and may, therefore,
be suggestive of why vegetation changed generally there.

Relict communities occur where local edaphic, topographic, or
biotic factors differ sufficiently to compensate for the effects of
environmental conditions obtaining generally. Thus altitude, ex-
posure, or soil may provide locally unusual moisture conditions.
Ridges, streams, and lakes may constitute barriers to fire. Peculi-
arities of drainage may result in swamps, bogs, and low flood

312 THE STUDY OF PLANT COMMUNITIES ■ Chapter XI

plains. Any such condition may be effective in maintaining relict
communities, which, in terms of climate, could not be anticipated.
They are the relicts indicative of shifts of climax and climate over
long periods.

FlG. 168. Effect of grazing on mixed prairie in central Colorado. Short
grass, to left of fence, is typical over much of the region but, where cattle
are excluded, to right, mixed prairie develops.— Photo by R. B. Livingston.

A quite different kind of relict is one that is maintained by man,
purposely or otherwise, after he has destroyed or modified the
picture of climax generally. Overgrazing, cultivation, and lumber-
ing have destroyed or modified climax over extensive areas to such
a degree that its recognition and interpretation, even though its
destruction was within historic time, are dependent upon rem-

CLIMAX COMMUNITIES : SHIFTS WITH TIME

313

nants of the former vegetation. Such relicts may be found in
fence rows, along railroad right-of-ways, in old cemeteries, and
in any areas long undisturbed and may yield much information
about the past. The deliberately protected areas, such as game and
wildlife preserves, natural areas, Indian reservations, and national
parks, offer still more possibilities because of their extent, fre-
quently included virgin areas, and relative permanence.

FlG. 169. Postclimax community of ponderosa pine occurring as an iso-
lated island in sagebrush desert wherever the special local soil conditions exist
in Nevada. Often disjunct from nearest ponderosa pine forest of the Sierra
by fifty miles or more .— Photo by W. D. Billings.22

Relicts related to climatic change are most abundant in the
transitions from one climax to another but may likewise be found
well within the general range of a climax, provided the local con-
ditions are present that maintain the necessary compensating fac-
tors. Usually the local conditions are a result of topography,
which, through its effects on precipitation, drainage, tempera-
ture, and air movements, permits the relict to survive. The result-
ing relict communities are the postclimaxes and preclimaxes pre-
viously discussed in detail. With an understanding of the concept
of postclimax and preclimax, their presence greatly simplifies the
interpretation of shifts of climate and climax in the past. The pres-
ent condition of the relict, if free from disturbance, may furnish

314 THE STUDY OF PLANT COMMUNITIES • Chapter XI

strong evidence of the present degree of climatic stability. Judg-
ment of such evidence must, of necessity, be tempered by what is
known of climatic cycles. In parts of the West, precipitation may
be several times as great during a period of sunspot minimum as
that during sunspot maximum. The condition of vegetation, par-
ticularly in relict communities, must be interpreted accordingly.

GENERAL REFERENCES

S. A. CAIN. Pollen Analysis as a Paleo-Ecological Research Method.
S. A. CAIN. Foundations of Plant Geography.

F. E. CLEMENTS. The Relict Method in Dynamic Ecology.

A. P DACKNOWSKI. Peat Deposits and Their Evidence of Climatic Change.

G. ERDTMAN. An Introduction to Pollen Analysis.

W S. Glock. Principles and Methods of Tree-Ring Analysis.
R B. SEARS. Climatic Interpretation of Postglacial Pollen Deposits in North
America.

Part 5 • Practical Consider ati

CHAPTER XII

APPLIED ECOLOGY

Man is rapidly becoming the earth's dominant organism. To an
increasing extent, natural communities survive because he tolerates
them, are modified to suit his purposes or fancy, or are destroyed,
sometimes through his carelessness, but usually so that land may
be used for agriculture, industry, or other activities. His domi-
nance is of a different order from that characteristic of com-
munities in nature, for, with his knowledge and technology, his
activities are often so extreme and so rapid that their effects are
like those of a series of catastrophic natural events. Thus he may
not only destroy or modify natural communities, but he may also
frequently modify the environment to a great extent. Suggestive
of a different form of environmental modification are the recent
experiments with rain-making by the use of dry ice. All this is
necessary from our modern point of view and will continue, per-
haps at an accelerated rate, as populations increase and the earth
is more completely occupied and used.

Natural communities and their environments, particularly the
soil, are natural resources. When they are destroyed or modified,
they may reappear only after a Jong period of time or, with ex-
treme disturbance, this may even be impossible. It becomes in-
creasingly apparent that future generations may require these
natural resources and likewise that man has been most wasteful of
them, especially in modern times. A problem today, which will
become greater in the future, is that of how to use such natural

resources to the fullest extent without jeopardizing their con-
tinued availability for future needs. The problem is fundamentally
ecological. Its solution depends upon the comprehension and ap-
plication of ecological principles.

315

316 THE STUDY OF PLANT COMMUNITIES • Chapter XII

Since man is becoming the dominant organism and also is
gifted with thought processes, his dominance should be such that
he turns natural laws to his advantage or, at least, does not permit
them to work against him. It is in this connection that applied
ecology becomes useful. The characteristics and distribution of
natural communities, the nature of the environment, and the inter-
relationships of organisms and environment are subject to natural
laws, which the ecologist seeks to recognize and verify. The more
completely the pattern of these interrelated processes is under-
stood, the greater the probability that man will remain a per-
manent dominant, assuming that he restricts his activities to the
limits of these laws. Only if biological laws are recognized in full
can we hope to rebuild the natural resources we have destroyed,
or even maintain those still available to us.

If we knew the ecology of all natural vegetation and that of all
crop plants, strong recommendations for land use could be made
in terms of its greatest contribution to society. Not only could
agricultural, forestry, and grazing lands be positively recognized,
but the details of management for maximum continuous production
could be recommended with certainty. Quite obviously, ecologi-
cal knowledge has not accumulated to this extent. The ecology of
natural vegetation is still inadequately known, and the ecology of
cultivated plants has not been sufficiently studied. If the ecologist
is to contribute successfully to the direction of man's activities as
a dominant, there is still much that must be learned. On the other
hand, even though knowledge is incomplete, ecology has much
to contribute that has not been fully utilized in applied fields.
What is known should be applied when man destroys or modifies
natural communities. Much progress has been made in the use of
ecological principles in several fields, but their potential applica-
tion is still great.

FORESTRY

The early history of lumbering in North America indicates, on
the part of lumbermen, a complete disregard for forests as a
natural resource and little concern for the future. Foresters have
long been conscious of this improdigal attitude although until re-
cently they were usually unable to change the lumberman's
methods or point of view. Through the years, forestry has be-

APPLIED ECOLOGY 317

come a respected profession as the necessity for scientific manage-
ment has become apparent. An important part of a forester's train-
ing is forest ecology, or silvics, in which he learns the scientific
background upon which silvicultural practices are based.

A generally accepted definition of silviculture states that it is
that branch of forestry dealing with the establishment, develop-
ment, care, and reproduction of stands of timber.254 More often
than not, the silviculturist aims to control the establishment and
development of forests so that they will be made up predomi-
nately of economically desirable species or so that merchantable
timber will be produced in a minimum of time. Or, he may be
interested in results not directly related to the production of
lumber. Cultural operations may point to erosion control, water-
shed protection, dune stabilization, game encouragement, or rec-
reational purposes,244 or, if in the West, to a better balance be-
tween timber production, watershed control, and use of the forest
for range purposes. If his methods are scientific, they will be
based upon reasons derived from silvics. Consequently, the more
completely forest ecology is understood, the more successful
should be its application in silviculture.

Since the practice of silviculture almost invariably involves at-
tempts to control forest communities and their development, a
knowledge of successional trends and the climax of the region is
all important. Knowing the principles of succession, it should be
obvious that the simplest form of management would be one that
least modifies the natural development of vegetation. To main-
tain a successional community indefinitely requires considerable
effort, if it can be done at all, but the nearer the desired forest type
is to the climax, the easier it should be to maintain it. These may
seem to be obvious generalizations, but they have not been, and
are not, fully appreciated or applied.

In the past, artificial forest types have been attempted under a
great variety of conditions. Species have been planted outside the
limits of their natural ranges, even including several introduced
from other continents. Often such trees are grown in pure stands
or, if not, then in combination with native species to make quite
unnatural communities. Even more common have been the at-
tempts to grow species on sites to which they are not naturally

318 THE STUDY OF PLANT COMMUNITIES * Chapter XII

adapted. The situation in New England is illustrative. Here the
original forest has long been gone, and reforestation and silvi-
cultural programs have been in progress for some time. Introduc-
tions include Scotch pine, European larch, and Norway spruce
from Europe, and white spruce from the northern conifer forest.
Red pine and white pine have been grown at the fringe of their
range in pure stands on rich, heavy soils instead of the sandy soils
on which they naturally occur.

The production of artificial forest types in New England can,
as a whole, be described as unsuccessful. S. H. Spurr and C. A.
Cline,245 pleading for the application of ecological principles, say
that older trees are often of poor form, and growth is likely to
decline sharply in later years. Very few artificial stands have been
profitably brought to maturity. Furthermore, these types are
especially susceptible to damage— from insects and other animals,
from disease, and from the elements. Norway spruce is severely
attacked by the white pine weevil; exotic larch plantations may
be severely damaged by the porcupine and squirrel; red pine,
south of its natural range, is particularly susceptible to Tympanis
canker and to attacks by the European pine-shoot moth; crooked-
ness of Scotch pine has been attributed to frost damage; weevils
do more damage to white pine on heavy than on light soils. These
authors admit that eventually, if sufficient knowledge is acquired,
artificial types may be grown successfully. For the present, they
cannot be recommended for New England because of previous
lack of success, the risk involved, and the high cost of production.
Probably similar generalizations can be made for most of the
forest regions of North America but with less evidence because
there has not been as much experimenting elsewhere. Although
forest species have been successfully introduced into new areas,
as, for example, the eucalyptus into California, the results in New
England are suggestive that such experimenting might be of
dubious value and certainly would not yield the necessary in-
formation except at great cost over a long period of years.

If only natural forest communities are to be the objective, there
are two general types to be considered. Silviculture is usually
given consideration only after the old forests have been destroyed
and, not uncommonly, after much of the land has been used for
agriculture and subsequently abandoned. Under these conditions,

APPLIED ECOLOGY

319

FlG. 170. Eight-year-old plantations of pine on the same soil type (Duke
Forest, Piedmont of N. C.) to compare growth of northern species, (1) red
pine, and (2) white pine, with that of native loblolly pine (3). The pictures
speak for themselves.— Photos by W. R. Boggess.

320 THE STUDY OF PLANT COMMUNITIES * Chapter XII

the abandoned land supports various early stages of secondary
succession, and cutover land is in late successional or subclimax
forest. The problem then becomes one of cultural practices de-
signed (1) to maintain the temporary forests of successional na-
ture or (2) to permit stands to develop to climax or near-climax
conditions.

The relatively short-lived successional communities often in-
clude as dominants the most valuable trees (e.g., pine where hard-
woods are climax) and, because of their rapid growth, the most
desirable commercial species growing in the region. But, because
of their successional position, when these species are removed,
they are replaced by other species, representing later stages of
succession, whose seedlings were there and released by the cut-
ting. The problem of maintaining dominance of such temporary
species has been given much study, but it is by no means solved.
Without expensive cultural operations, usually including planting
and periodic weeding, these temporary types cannot be main-
tained indefinitely. Even though the productiveness of a desired
species in a stand may be extended by various types of cutting and
treatment, its replacement is inevitable. Almost invariably, the
succeeding stand tends to be nearer the climax than its predecessor
and will include a higher proportion of economically less desir-
able species. Where successional species are fire resistant, there is
the possibility of using controlled burning to hold back succession
and maintain dominance of the temporary type. Under these
conditions a temporary type could be cut selectively and provide
a continuous yield. The merits of the method have been argued
and are being tested for the longleaf pine forests of the coastal
plain of the southeast.

The alternative would be to allow all forest land to develop to-
ward the climax or at least to near-climax conditions. Once estab-
lished, such forest would require a minimum of silvicultural
attention. Continuous production would be assured, and with
judicious selection of species for cutting, undoubtedly the pro-
portions of desirable and undesirable species could be controlled.
Additionally, permitting natural development of stands should
result in a distribution of species in the habitats to which they are
best adapted. Different conditions of soil, exposure, and moisture

APPLIED ECOLOGY 321

would support stands of different composition, but presumably
these species would be making their best growth although a mini-
mum of management would be involved. This is not to imply that
silviculture is unnecessary. For example, artificial planting is fre-
quently economically justifiable since it assures uniform stocking
and even-aged stands and may speed stand development by several
years. If there are few seed sources of desirable species, succession
may be so long delayed, by shrub stages perhaps, that planting be-
comes a necessity.

Silviculture is usually desirable and sometimes a necessity, but
it should be emphasized that its practices, to be most effective,
should be governed by ecological knowledge. The less cultural
practices tend to modify the natural trends of succession, and the
more nearly the desired forest is to the climax of the region, the
less the effort and expense there will be in developing and main-
taining it. Here is an economic reason for learning the nature of
virgin forest wherever it still remains and for determining all that
is possible of its variations with habitat. Similarly, successional
trends must be known in detail for every major soil type and situa-
tion if cultural practices are to be adjusted accordingly. Secondary
successions are of major importance these days, and they can be
worked out for any region. Climax forest in virgin condition is
rapidly disappearing and usually only remnants remain for study.
Their characteristics should be recorded at every opportunity.
When possible, representative portions of these virgin forests
should be saved intact for future study.

RANGE MANAGEMENT^

The objective of range management is to produce the highest
possible forage yield while the condition of the range is maintained
or actually improved. To this end, the methods of ecology have
been used to such an extent that range management is largely ap-
plied ecology, and just as silvics is the basis of silviculture, so is
range ecology the basis of range management.

Range ecology has, on the one Hand, concerned itself with the
purely ecological concepts of regional climaxes with grazing value
and the patterns of succession for each. On the other hand, there
has been the practical consideration of the quality and type of

322 THE STUDY OF PLANT COMMUNITIES * Chapter XII

•^ajfi&ijfc^^^^^*

^** >* «**

FIG. 171. Illustrations of blue grama-oak savannah range that tell their
own story of good and poor range management. (Above) "One of the finest
demonstrations of range and livestock management in the southwest!' (Be-
low) Range depleted by overuse and poor management. Note differences in
condition of cattle, amount of forage, ground cover, and erosion.— U. S.
Forest Service.

APPLIED ECOLOGY

323

forage provided by each of the communities and of how they may
be controlled or modified to advantage. Only suggestions of the
nature of the research on these problems can be given here, but
they should indicate the degree to which ecology is contributing
to the solution of range problems.

FlG. 172. To permit grazing to continue until range is entirely depleted
and gullying has reached such extremes is obvious mismanagement, but it
happens all too frequently. Note absence of gullies under protection of oak
tree.— U. S. Forest Service.

The seasonal variations of major species have been studied in
terms of grazing value. Competitive relationships of grasses and
forbs (associated herbs) have been investigated as well as their
relative palatability. The effects of grazing on community struc-
ture have been given much attention, particularly with regard to
criteria for the recognition of excessive use and the time and con-
ditions necessary for recovery to normal. As a result, the carrying
capacity of many forage types is well known, even for different
seasons of the year. With regard to range condition and carrying
capacity, the effects of rodents have been studied as well as the
effects of predators upon the rodents. Effects of drought have
been given much attention as well as the rates with which ranges
recover from drought, and, in this connection, the water require-
ments of important individual species have been determined. In

324 THE STUDY OF PLANT COMMUNITIES • Chapter XII

the consideration of water, the effects of different types of cover
on runoff, flooding, erosion, and water supplies have been studied
in detail. In attempts to rebuild depleted and eroded ranges, there
have been studies of artificial seeding and planting to speed recov-
ery. As in forestry, numerous foreign species have been tested with
some successes (e.g., crested wheat grass) in an attempt to improve
conditions.

'9n

*^f

.JKf v^^

FlG. 173. Two years before, this Idaho range supported only Wyethia and
sage. Seeding with timothy, smooth brome, and clover, and protection for
one year produced this abundance of forage at the end of the first grazing
season.— U. S. Forest Service.

Because grazing is a part of every question in range ecology, the
exclosure method is an important technique in range research.
Exclosures are especially useful for testing experimental condi-
tions, but they, or equivalent isolated areas, are likewise necessary
for determining:- the nature of climax and related successional
communities. In experimental studies, exclosures, in combination
with grazed areas around them, are one of the better means of de-
termining the effects of conditions in progress on that range. If
causes are to be investigated, they are tested separately, each with
its controlled treatment, on individual plots within an exclosure.
Such treatments may include clipping (for grazing), burning,
trampling, seeding, etc. As indicated earlier, the installation of

APPLIED ECOLOGY 325

exclosures of sufficient size, which will keep out rodents and yet
will not alter microclimate, presents numerous difficulties. Conse-
quently comparative studies on ranges supporting different, but
known, animal units are coming into use whenever possible be-
cause they do not require exclosures.

When the results of such studies are evaluated and expressed in
general terms, it becomes apparent that several principles have
been established that appear to be universally applicable.59 From
an ecological point of view, these principles, determined by ex-
periment, would seem to be self-evident since they conform to
ecological theory. It must be remembered, however, that these
things were originally theory and now can be stated as principles
supported by experimental evidence. The testing was necessary to
establish them as tried bases for range management. In grasslands,
no less than elsewhere, succession is operational, and all trends
constantly proceed toward the climax unless they are modified by
disturbance or are held in check by an unfavorable swing of cli-
mate, as during a series of dry years. Grassland is a climatic life
form, which maintains itself in the absence of disturbance and
which, if destroyed, reappears when the disturbance is removed.
All evidence indicates that perennial grasses become dominant and
eliminate annual grasses, forbs, and shrubs in the absence of graz-
ing, fire, or similar destructive agencies. The grasses of a particular
climax are adapted to its climate and usually have an advantage in
terms of competition over introduced ones.

From the above, it becomes apparent that, as in forestry, prac-
tices of management which least disturb the natural balance of
grassland and its environment are most desirable. Those that take
into consideration the trends of succession and local climax are
likely to be most successful at the same time that they require a
minimum of expended effort. Although a few exotic species have
proved to be easier to propagate than native ones, the introduction
of foreign species for range improvement or erosion control is
likely to be unsatisfactory unless those species are to be given
extra care or special cultural conditions. In fact, there is evidence
that seeding of native species should be done only with locally
produced seeds since the species may consist of geographic physi-
ological races.

326 THE STUDY OF PLANT COMMUNITIES • Chapter XII

The establishment of general principles is being followed by
more and more intensive studies of local variations in communities
and environments. The productivity of most range lands has been
reduced by man's domestic animals coupled with seasons of un-
favorable climate, and to rebuild ranges to a higher level of pro-
ductivity will require an understanding of the special conditions
of local areas as well as the broad principles for the region. Our
public lands in the West, most of which are grazed, have been di-
vided, for research and administration, in a fashion that suggests
a natural application of the above. Several grazing regions are
designated, which correspond to the major differences in the
grassland and scrub formations. These in turn are divided into
several districts, which represent local variations in dominants and
environment. Application of the general principles is possible for
regional administration and management, but local application
must be modified in terms of detailed local studies.

AGRICULTURE

If a crop is planted and grown successfully, it follows that the
methods applied, within the general region, to the particular field
and for that season, were ecologically correct since cultivated
crops are as subject to ecological laws as are plants growing nat-
urally. Study of the ecology of cultivated plants has progressed
rapidly in recent years. It includes crop ecology, which is applied
ecology in the ordinary sense, and ecological crop geography,
which considers the effects of both physiological and economic
factors on production and distribution of crop plants.143 With
this addition of "social" factors to the physical and physiological
ones, the already complex environment becomes still more so, and
the crop ecologist must integrate his observations and conclusions
with additional fields. This phase of ecology is, as a whole, beyond
our consideration here, but it is appropriate to emphasize that
ecological principles are becoming a part of our way of thinking.
They should undoubtedly be given even greater attention in these
days of a planned economy, which affects us all.

Crop Ecology.— The cultivated plant is as subject to ecological
law as a native one, and, consequently, there is as much ecology
to be studied in a corn, tobacco, or cotton field as there is in a

APPLIED ECOLOGY 327

forest. To be sure, largely by trial and error, the farmer has learned
to grow crops so that they give a reasonable return for his labor.
But, on the whole, this has been done at the expense of the soil as
a natural resource. The natural fertility of most of our soils is
largely depleted, erosion has ruined thousands of acres and re-
duced the productivity of many more, and water tables have been
lowered to such an extent that crops in areas with rainfall suffi-
cient for hardwood forest are suffering during dry spells as much
as they would in grassland climate. Thanks to increased knowledge
of fertilizers, the development of productive hybrid strains of
various crop plants, and modern mechanized methods, our yields
have steadily increased, but this cannot proceed indefinitely, espe-
cially since much of the increase in yield has resulted in further
depletion of the soil.

To counteract the inevitable downward trend of productivity,
soil conservation and erosion control are receiving greater atten-
tion. Increased knowledge of crop ecology is imperative so that
the highest yielding species will be grown under the proper con-
ditions of cultivation and on the right sites. If possible, yields must
be maintained at high levels at the same time that soils are im-
proved rather than being depleted. The ecology of weeds, pests,
and diseases must be studied so that the depredations of these prod-
ucts of cultivation may be held in check effectively. These things
are not being neglected by agronomists and horticulturists, but
there are special contributions that can be made if the investigator
has the ecological point of view.

Land Use.— It has been customary to clear all workable land for
agriculture, permit plowland to revert to pasture only when it
becomes unprofitable, and permit pasture in turn to revert to for-
est only under the same conditions. It may be desirable to reverse
this procedure completely. Perhaps the soundest ecological ap-
portionment of the landscape would be represented by a minimum
of carefully selected, skillfully operated plowland with a max-
imum of natural vegetation. Where this natural vegetation consists
of grassland, regulated pasture is an aspect of its normal develop-
ment; where it consists of forest, it should be scrupulously pro-
tected against grazing, and whatever pastures are required should
be handled with the same measure of skill that has been suggested

328 THE STUDY OF PLANT COMMUNITIES ■ Chapter XII

FlG. 174. Once-fertile farm land that has been unnecessarily destroyed by
surface erosion and gullying because of lack of concern (note that straight-
row cultivation still prevails) and lack of understanding. Perhaps this area
should have been put into forest long since. If it had been, it would still be
valuable.— U. 5. Soil Conservation Service.

for the plowland.223

Maintaining stands of natural vegetation provides areas for eco-
logical comparison and diagnosis, insures that soil is being rebuilt
and retained, provides organic matter, insures a regulation of mois-
ture conditions that man cannot duplicate, and provides food and
shelter for wildlife, which may be significant in reducing crop
pests.

The planning of such land use should be, in so far as possible,
based upon ecological principles as related to soil, topography,
exposure, and drainage in terms of the climate and cultivated crops
it will support. Special land-use problems arise on hilly land, which
need not necessarily be unproductive. Ecological studies of hill-
culture172 are showing how some such lands may be used to grow
orchards, vineyards, pasture, and other crops without depletion
or erosion of the soil.* Where streams occur, it has been shown

*Much of the following discussion of applied ecologv in agriculture is
adapted from an unpublished report by the Committee on Applied Ecology
of the Ecological Society of America, 1944.

APPLIED ECOLOGY

329

that artificial fishponds can be a profitable investment. The eco-
logical problems to be solved for such ponds include sizes and
depths for different climates, drainage, amount of available water
and necessary aeration, rate of silting under different conditions,
fish food relations, kinds and amounts of fertilizer necessary, kinds
of fish, and rate of stocking. Marshes might be retained and im-

FlG. 175. A half-acre farm pond in West Virginia of the type being wide-
ly installed for food production and recreation.— U. S. Soil Conservation
Service.

proved for muskrat production, but, again, the practical problems,
largely ecological, have not been sufficiently explored. Stream
margins create other land-use problems. Usually they are grazed,
and, as a result, they erode. The species that would appear under
protection should be known, as well as the most desirable species
for checking erosion. In many sections, planted hedges and field
border plantings are being recommended on the unproductive
margins of fields to reduce erosion and provide cover for wildlife.
The ecology of the planted species must be known as well as its
effects on the crop beside it. Also the ecology of the insects, birds,
and mammals of these margins must be known. Are they desirable,
beneficial, or are they harmful to desirable species?

Land Management.— The operations by which land is prepared
for crops, their planting, harvest, and use are known as land man-
agement. For greatest efficiency, good land management must
parallel good land use. These are arts but, today, arts requiring all

330 THE STUDY OF PLANT COMMUNITIES ■ Chapter XII

FlG. 176. This eroded stream bank in Wisconsin was graded, layed with
willow poles, and planted with a few willow sprouts. Only two seasons were
required to produce the growth shown in the second picture where under-
cutting is effectively stopped and shelter is provided for wildlife.— U. S. Soil
Conservation Service.

APPLIED ECOLOGY

331

FlG. 177. Waste field-margins such as the fourteen-foot strip (1) aban-
doned because of root competition and erosion can be made useful. (2)
Lespedeza bicolor (tall) and L. sericea planted in strips are holding the mar-
gin stable and producing food and cover for small game.— U. S. Soil Con-
servation Service.

the help of science possible.226 A farm planted year after year to
wheat or cotton does not, even with fertilizer, conform to the
balances that occur in nature. Well-managed fields may seem to

332 THE STUDY OF PLANT COMMUNITIES ■ Chapter XII

approach a condition of balance as a result of rests with rotation
pasture, the use of legumes, and the addition of fertilizer. Yields
may be high and sustained, soil may not erode, and all appear to
be at its best.

In terms of natural vegetation, however, our modern methods
of land management may be questioned. Cultivation produces

FlG. 178. A simple illustration of improper management. The amount of
runoff on this slope means leaching and erosion. Certainly the rows should
not have been put in up and down the hill, and perhaps, without terraces,
clean cultivation should be ruled out on this field. — t/. S. Soil Conservation
Service.

conditions similar to those in early stages of succession, conditions
that in nature would be temporary and soon change in the direc-
tion of climax. We must have crops, but, if climax vegetation
utilizes natural conditions most effectively— and that seems reason-
able—are our methods of cultivation the best we can use for ob-
taining our crops? Is our method of deep plowing, with destruc-
tion of soil structure, best under all conditions? Should all crops

APPLIED ECOLOGY

333

FlG. 179. Deciduous forest farm wood lots, pastured (above) and not pas-
tured (below), which illustrate the effects of browsing and trampling on
reproduction and general forest condition.— U. S. Forest Service.

be cultivated clean and all organic matter be turned into the soil?
Might mixed crops producing a complete cover as in nature not
be more desirable? Perhaps we have gone too far in producing

334 the study of plant communities ■ Chapter XII

unnatural conditions. The artificial environment of cultivation re-
sults in soil erosion, a modified soil flora and fauna, and changes in
water relations. Also we have more diseases of crop plants and
more insect pests than ever before.

These are ecological problems. Intelligent land use minimizes
some of them. Practices like contour plowing, terracing, and strip
cropping are moves in the direction of reducing them. But when
the ecology of crop plants is studied further, especially in terms
of natural vegetation, some of our methods of use and management
may require revision.

Pasture Problems.— Above, it was suggested that the same atten-
tion to management should be given to pastures as to plowed land.
This would be a reversal of the usual point of view since pastures
are, more often than not, largely on the poorest land and are given
little or no attention. With the steady expansion of dairying, espe-
cially into sections of the country where adequate pastures do not

FlG. 180 (1). An Indiana field after fall plowing showing severe erosion.
Picture taken when it was decided to retire field to permanent pasture with
contour furrows.

APPLIED ECOLOGY

335

Fig. 180 (2). The next year, after gully-control work, this excellent
planted pasture had taken over, the soil was stabilized and the field saved for
long-continued usefulness.— Both photos by U. S. Soil Conservation Service.

produce themselves, the need for pasture ecology increases. The
necessity for seeding is now widely accepted. Many species have
been tested for palatability, yield, food value, and soil-building
properties. Growing pastures is still, however, largely a hit-or-
miss affair that requires much more study. Regional pasture ecol-
ogy has not progressed as far as range ecology. There is much yet
to be learned, tested, and put into practice. The implementation
of such a program will be difficult in many sections where pastures
are not generally recognized as a crop to be managed like any
other.

An illustration of the misconceptions regarding pasture is the
common practice of including the farm wood lot in the pastured
area although it provides little more than browsing, which sup-
plements feed during off seasons. To the ecologist, it is obvious
that this is at the expense of seedlings and ground cover and that
it will result in stand deterioration.160 Silviculturists have shown

336 THE STUDY OF PLANT COMMUNITIES ■ Chapter XII

that properly managed wood lots can yield as great a return as
any average farm acreage, but the wood lot pasture persists. A
study of maple groves in Ohio225 showed that in three years the
elimination of grazing resulted in an increased yield of maple
syrup, worth more than twice what the rental for pasture would
have been. At the same time, the condition of the stand was no-
ticeably improved. As more such information is accumulated83 it
is to be hoped that its application will follow.

Regional pasture studies must be continued so that both species
and their culture can be recommended with confidence for cli-
mate, soils, and land management policies as they occur. To obtain
such results, it would appear that ecological methods should be the
most promising.

Insect Problems.— The relationship between land-management
practices and insect populations is inadequately known.116
Whether insect pests will increase or decrease with strip-cropping
or particular crop rotations cannot be said with certainty. Prob-
ably more complex are the relationships of insect populations to
the birds and mammals that will appear in response to such con-
servation practices as cover crops, hedges, and field border plant-
ings. Whenever the acreage of a cultivated species is increased
extensively in an area or a new species is introduced for special
purposes such as erosion control, insects may appear with it or
abruptly increase in numbers to pest proportions. Such relation-
ships and innumerable others need more study. The possibilities
for applied insect ecology in agriculture and forestry are almost
unlimited.

Rodent Problems.— Especially for range lands, ecological knowl-
edge of rodents is still inadequate. In spite of this, rodent control
has been attempted in these areas for years. More should be known
of the effects upon rodent populations of kinds and degree of
grazing as well as what effects the various rodent-control measures
have on the condition of the range. With the latter, it should be
possible to say what percentage of a rodent population can be
destroyed by a control measure, how long before the surviving
population will return to normal, and to what extent species move
in from untreated areas. Complicating the above problems is the
usually cyclical fluctuation of most rodent populations and the

APPLIED ECOLOGY 337

obvious desirability for adjusting control to these natural fluctua-
tions. Other suggested ecological problems are the relationship of
rodents to reseeding, succession, and climax in range land, and
their numbers and effects upon orchards when managed with
cover crops.

Weeds.— The occurrence of weeds as a result of land use and
their control by cultural practices have received far less attention
than control by direct, aggressive means. Yet cultural control or
control as a result of good land management is likely to be the
most permanent and least costly. Certainly the weed problem has
not been reduced by centuries of cultivation, mowing, and burn-
ing. Even modern ''hormone" sprays are no panacea.208 If progress
is to be made, the autecology of the principal weed species must
be studied in detail. If, then, the effects of various types of land
use and management upon the occurrence of specific weed species
is learned, there is a reasonable possibility that ecological controls
could be recommended that would reduce the weed problem,
under certain situations at least.

CONSERVATION

The problems of conservation are extremely diverse, including
as they do such things as soil and soil water, wildlife of all kinds,
and aesthetic considerations. All that we have discussed of applied
ecology could be classified under the general heading of conserva-
tion. The field is so broad as to require specialists of all kinds in
its management, but this, of all fields, requires training to see each
problem in the light of others. Nowhere can the ecological point
of view be more effectively applied.250

To illustrate the limited effectiveness of specialization without
ecological appreciation, witness such operations as have been
known to take place almost simultaneously on public lands : a
road crew cutting a grade in a clay bank so as permanently to roil
a trout stream that another crew is improving with dams and
shelters; a silvicultural crew felling wolf trees and border shrub-
bery necessary for game food; a roadside cleanup crew burning
all fallen oak fuel available for fireplaces that are being built by a
recreation crew; a planting crew setting out pines in the only
open fields available to deer and partridge; and a fire-line crew

338 THE STUDY OF PLANT COMMUNITIES ' Chapter XII

cutting and burning all hollow snags on a wildlife refuge.153 Such
conflicting activities have not been uncommon in the name of
conservation. Some government agencies have spent millions for
flooding marshes and improving them for wildlife while other
agencies were attempting to drain marshes of questionable agri-
cultural value. Great dams have been built for reclamation pur-

FlG. 181. The deposition of silt and sand behind a dam in this fashion de-
feats its purpose of water storage and reduces the efficiency as a source of
hydroelectric power.— U. S. Soil Conservation Service.

poses, but the watersheds above them have been ignored.227 With
continued lumbering and grazing, the reservoirs are silting in so
rapidly that the usefulness of the dams promises to be short-lived.
To assure integration of such activities may not require a "declara-
tion of interdependence"250 but certainly the recognition of the
interdependence of biological phenomena is necessary. This end
will certainly be served if those responsible are ecologically trained
or have an ecological point of view regardless of their special in-
terests.

Soil Conservation.— The recognition of soil conservation as a
national problem is of recent origin. The Soil Conservation Service
was made a permanent bureau of the U. S. Department of Agri-
culture in 1935 athough it originated as the Soil Erosion Service
in the Department of the Interior in 1933. Since then great prog-

APPLIED ECOLOGY 339

ress has been made in educating the public to the need for a con-
tinuous program of conservation, and soil conservation as a science
has developed rapidly. The scope of the field and the problems
involved have been admirably summarized in various publica-
tions.18' 136

Early publicity by soil conservationists was essentially a plea to
save our irreplaceable land, a great deal of which was already per-
manently lost and much of which is in the process of being ruined.
More recently, the emphasis has been upon rebuilding lands that
have deteriorated. The modern philosophy considers soils, like
forests, to be natural resources that are renewable and, therefore,
subject to management that will give a sustained yield over an
indefinite period of time.174 Such a program is, of course, as justifi-
able as the original, which aimed primarily at erosion control. It
indicates that the conservation program has been successful and is
maturing.

Soil conservation is, therefore, more than erosion control. It
also involves the retention of water, especially on slopes, and its
utilization to best advantage. At the same time, it aims to maintain
or increase soil fertility and productivity. Thus soil conservation
is merely the practice of agriculture in the best possible way, and
we have already suggested how the ecological approach to such
problems is most likely to be successful.

Not all the various measures successfully introduced for erosion
control and soil building are applicable everywhere but must be
adjusted in terms of soil types and climate. However, certain gen-
eralizations can be made which have wide application and whose
special use or desirability often must be determined by a knowl-
edge of local ecology. Vegetative- cover is the most effective means
of checking erosion. This raises questions as to what cover is de-
sirable or possible under different conditions, where it should be
permanent, and when it should be of native vegetation. These
problems are related to strip-cropping, gully control, cover crops,
and decisions to cultivate hilly land, put it into pasture, or plant it
to forest. It is now assumed that the control of erosion will pay
dividends only when proper crop rotations and fertilizing prac-
tices are followed. The interrelationships must be known for every
crop and region.

340 THE STUDY OF PLANT COMMUNITIES ' Chapter XII

Much advance has been made in cultural practice. Contour
plowing, in which cultivation follows lines of equal elevation, is
becoming steadily more common. In many areas, strip-cropping
is an additional control, in which clean-cultivated crops are
planted between strips of cover crops, such as legumes, which
retard runoff and hold soil. A further necessity on contoured

FlG. 182. Aerial view showing strip-cropping of terraces that follow con-
tours. Erosion is checked, much water is retained, and what runs off is di-
rected to a sodded runaway channel. Such elaborate operations may require
co-operation of several landowners. In this instance, two farms are involved.
— U. S. Soil Conservation Service.

slopes may be terraces, which are ridges so placed that they catch
and hold water in a channel behind themselves and thus check
runoff and cause water to soak in. In special instances, deep fur-
rows are maintained (listing) in which water and snow are held
and crops are planted in the bottom of these troughs. Basin listing
is done on some soils with special machinery that shapes these
troughs with cross dams at regular intervals further to reduce
runoff. It has been shown that wind erosion can be reduced by

APPLIED ECOLOGY 341

"stubble mulching" in which subsurface tillage keeps old organic
debris on the surface. Windbreaks of various kinds are known to
be effective also.

All these are examples of modern practices that are proving
effective under special conditions. They are not by any means
new, since they have been reported in various forms far back in
history. It is their application in the light of modern knowledge
that marks advance. The more complete the knowledge of all fac-
tors involved— crop, soil, climate— the greater the success of their
application in the future. The research programs continue, and the
kinds of investigations in progress are invariably ecological in
nature. Here is a list of a few of the projects being studied for a
single district :172

1. The effect of contouring corn, soybeans, and oats on
soil and water conservation

2. The effect of divergence of rows from the contour on
losses of soil and water

3. Cultural practices and methods of handling crop resi-
dues in relation to soil and water conservation and crop
yield

4. The effect of cover crops on the conservation of soil
and water and on crop yield

5. Investigations of soil moisture content under different
crops, cropping systems, and mechanical conservation
practices

6. Effect of crops and organic matter treatment on the
movement of water through the soil profile

Other studies include effects of cropping systems, crop rota-
tions, handling of crop residues, and management in terms of run-
off, yield, and soil properties.

Some special problems of soil conservation still requiring a
great deal of study are related to drainage of water-logged land
and swamps, irrigation of lands with insufficient water, clearing
of toxic salts from irrigated land and other lands not previously
cultivated.

Water Supply-— The conditions necessitating soil erosion con-
trol and the prevention of runoff of surface water are commonly

342 THE STUDY OF PLANT COMMUNITIES * Chapter XII

reflected in the general water supply. In many agricultural areas
with adequate rainfall, there are water problems that did not exist
at the time of settlement. Where once streams and springs were
abundant and flowed continuously, now they are intermittent,
and summer water supplies are often low. In Ohio, the water table,
as evident in well depths, is from fifteen to fifty feet lower than

FlG. 183. The type of dam and spillway being installed primarily for
water conservation. When full, this reservoir extends fifteen miles upstream
over an area of 10,000 acres. The flow from the dam can be controlled, there-
by providing constant flow during dry periods and reducing danger of flood-
ing with high water.— U. S. Soil Conservation Service.

originally. Floods appear to be more frequent and are certainly
more destructive than before. On the credit side, there are now no
malaria problems related to undrained swamps or typhoid epi-
demics resulting from improper city water supplies.223 The adverse
conditions result partially from the removal of natural vegetation
for agriculture. As much water falls today as before, but more of it
runs off rapidly. Thus summer drought and spring floods are par-
tially explainable.

There are other contributing factors. Roads, so important to the
farmer for transportation, likewise serve to drain off water from
his fields. This has been especially bad in the mid-western states
where all roads were originally laid out in an east-west, north-

APPLIED ECOLOGY

Ep||b& .,,vv...;.::

i*r*f*^

-
-

Fig. 184. A power-dam lake at the edge of a town in Minnesota as it
appeared in 1926 when it was extensively used for fishing and recreation. By
1936 excessive silting had left only a small channel. Watersheds above the
dam were improperly handled; timber was removed, slopes were cultivated
and few precautions were taken to prevent erosion.-L/. 5. Soil Conservation
Service.

344 THE STUDY OF PLANT COMMUNITIES • Chapter XII

south grid pattern of blocks, which disregarded topography and
provided a powerful system of artificial drainage. Also great drain-
age projects were instituted in the earlier days of agriculture, and
these, too, served to speed the removal of water.

The trend in concern over surface water proceeded from drain-
age projects to those dealing with flood control. Such concern is
still with us, and necessarily so, because of the destructiveness of
floods to both property and land, but, a new trend is now apparent
in the attempts to conserve, retain, and store water so that it may
be available when needed, so that water tables may be held at
higher levels, and so that flood waters may be controlled. Dams
and reservoirs are being constructed and watersheds are being
protected.

A recent factor in the lowering of water tables is the great in-
crease of use of water in industry and the rapid increase of air-
conditioning. Much of the water used for the latter is wasted
because it is not used for any other purpose. The lowering of the
water table by using water for this purpose has caused much con-
cern in large cities. Various legislation is aimed at controlling the
use of this natural resource. Most large users drill their own wells,
but this practice is being limited. In some cities, it is required that
the water must be forced back into the earth at the levels from
which it is drawn.

Our water supply is a natural resource just as are the others we
have discussed. When its availability is reduced, it affects agricul-
ture, industry, fish and game, recreation, and perhaps home use.
The trend is already in the direction of its conservation. Probably
it will go further. Ecological problems of many kinds will arise in
connection with control of water in streams and reservoirs, and
the effects upon water table levels. It is a part of all the applied
ecology we have discussed.

Another facet of the problem of water supply is its pollution by
industrial waste and sewage. Here again, there are innumerable
problems of an ecological nature. Their solution often requires the
co-operation of engineers, chemists, bacteriologists, and limnol-
ogists. As always, when such specialists are drawn together, their
success is greatest when they see their own fields in relation to the
whole. This is the ecological approach.

APPLIED ECOLOGY 345

Wildlife.— Like soil, water, and forests, our wildlife constitutes a
renewable natural resource, which, consequently, can be restored
or maintained even while it is used, if the use is a wise one. All of
these renewable resources are so intimately related that a program
for the conservation of one must necessarily consider the others
as well. This ecological point of view is fully appreciated by lead-
ers in wildlife management. It is also realized that, when man be-
comes the dominant organism, the management of soil, water,
forests, and grassland is inevitable— and wildlife, too, if it is to be
preserved.

If wildlife management is to be successful, man must know the
ecology of the species involved, whether they are fish, birds, or
game animals. Life cycles must be known, as must breeding habits,
food habits, and food chains, migration routes, preferred habitats,
diseases, predators, population trends, and the carrying capacities
of given habitats. Such complete information is not yet available.
"In its present state, wildlife management is an effort to apply to
urgent problems the ecological and biological data that are now
available, always with the consciousness that existing tools, meth-
ods, and processes may have to be discarded as new and better in-
formation becomes available!'100 Ecological knowledge is still woe-
fully incomplete for most of our wildlife, although information
accumulates steadily. As it accumulates, programs of management
increase in effectiveness.

The range of ecological problems related to wildlife manage-
ment is tremendous. The complexity of management can perhaps
be suggested by indicating some of the kinds of things that must
be taken into consideration. It would seem that, if food and cover
are provided for an organism, its needs should be satisfied. But, for
many species, the feeding habits are inadequately known. Cover
can be provided for some species but, under present conditions,
frequently only in localized areas. If that is true, it is not uncom-
mon for food problems to become complicated during the winter
months when the species tends to become concentrated on these
restricted areas. A population that is reasonable in summer may
become excessive in winter and result in death by starvation for
many individuals. Encouraging the increase of one species may be
detrimental to another one; consequently, individual species must

346 THE STUDY OF PLANT COMMUNITIES * Chapter XII

be studied in relation to others. In this connection, predation must
be considered from an ecological standpoint.

Species whose numbers have declined to extremely low levels
may be propagated under controlled conditions and then released,
but the cost is often excessive. Others may be taken from areas of
overpopulation and transported elsewhere to start a new popula-
tion. Such activities have sometimes been successful but in other
instances have failed because of factors that were not known or
understood. The ecology of the species and of the region must be
known. If it is known, there is a reasonable possibility that the
species can be encouraged to increase naturally at much less ex-
pense and trouble. The problems related to overpopulations of
protected species are no less complicated, the ideal being a condi-
tion in which natural propagation produces a constant popula-
tion supportable by the environment and perhaps an excess suffi-
cient to permit a reasonable take by the sportsman.

When it is realized that such problems and many more are in
the process of solution for big game, birds of all kinds, fur ani-
mals, fish, and other wildlife, it should be apparent that there is
much basic ecological work to be done that has possibilities of
application. The mistakes that have been made in wildlife manage-
ment have undoubtedly resulted more often from inadequate eco-
logical information rather than from lack of appreciation of how
such knowledge could be applied if it were available. Wildlife
management is applied ecology, and it will progress as basic eco-
logical knowledge becomes available and is integrated by wildlife
ecologists.

Game refuges provide a safeguard against lack of knowledge
and provide the opportunity for acquiring needed information.
Particularly, they insure that scarce or disappearing species do not
become extinct as some have in the past, for here they are pro-
tected and given every encouragement to increase. Usually such
refuges do not result in the restoration of a vanishing population.
They do, however, insure a continuous breeding stock from which
restoration may be made, and they give excellent opportunity for
the study of the species involved under relatively undisturbed '
conditions or under available conditions.101 A few such refuges
are still in near primitive condition and thus can provide much of

APPLIED ECOLOGY

347

the biological knowledge of habitat, vegetation, and wildlife that
must be learned to manage other refuges and ultimately the gen-
eral program of wildlife conservation. Other refuges provide the
testing grounds for management procedures as knowledge accu-
mulates.

/.>- r+T*

*&m

,**<***r

BfeWu ■■'■ ^■P^

-*^ .,

^^''~ — * 31

xrCwjr,-^- •■ 2SB&

:.«*?

FlG. 185. An unsightly, eroding road cut in Illinois and its stable appear-
ance three years after planting with trees that blend with topography and
native vegetation.— (7. 5. So/7 Conservation Service.

348 THE STUDY OF PLANT COMMUNITIES * Chapter XII

LANDSCAPING
The planning and planting of vegetation for home beautifica-
tion or in public parks or gardens involves aesthetic considerations
but likewise should be backed by an appreciation of the ecology
of the species involved. If plantings are not made in terms of the
requirements of the species used, they cannot be successful. Soil

FlG. 186. On such road-building projects erosion control must be given
serious and prompt attention. These great fills have been stabilized by me-
chanical means and have likewise been planted. If aesthetic considerations
have entered into the stabilization program, they are not yet apparent— U. S.
Forest Service.

texture and structure must be considered as they affect water
relations. Slope and exposure modify drainage and temperature
just as they do in natural environments. Tolerance of shade, light,
or extremes of temperature cannot be ignored when planning
artificial combinations of species. Some species must be planted in
moist places, some require full sunlight, some need to be partially
shaded. Competition and all the other factors affecting natural
communities operate among planted species as well. The same
factors that limit the ranges of natural communities operate to
limit the usable materials of landscape design for different sections
of the country. Landscaping is, therefore, most successful when
based upon ecological principles.

APPLIED ECOLOGY

349

Natural landscaping is a recent development resulting from
man's modern engineering activities, which drastically change to-
pography, drainage, and vegetation when he constructs modern
highways, dams, and airports. Great exposures of subsoil in cuts
and fills require cover and replanting not only for aesthetic rea-
sons but also to check erosion and slumping. It is to be expected
that engineers should give first consideration to the efficiency of
installation and use of a project under construction, but, when this

FIG. 187. The old and the modern manner of handling a road cut. Note
the gradual back slope, seeded surface, and shallow, sodded runoff channel,
all designed to check erosion.— U. S. Soil Conservation Service.

350 THE STUDY OF PLANT COMMUNITIES * Chapter XII

has been the only concern, after effects on drainage and erosion
have frequently created serious problems. Not only has natural
beauty been destroyed unnecessarily at times, but extensive ex-
panses of bare soil, in fills and cuts, have been left for nature to
recover and stabilize. The re-establishment of natural vegetation is
often impossible before erosion and slumping cause disruption of
drainage, road blocks, and similar difficulties. Consequently, stabil-
ization must be provided for through artificial means and by seed-
ing and planting. The problem is intensified by the infertility of
the subsoil, upon which few things will grow. Although the first
concern should be stabilization, there should be consideration of
succession and the possibility of harmonizing the developing vege-
tation with that of the surrounding terrain.

In addition to large cuts and fills along mountain highways,
there are problems of maintaining road shoulders, ditches, and
spillways. Certainly not all is known about the best species for
such purposes under all conditions. Also the natural beauty de-
stroyed by a new right-of-way need not be permanently lost.
With a minimum of management it would seem that native species
could be encouraged to provide cover and beauty, especially along
the new express highways, which are increasing in number. It
does not seem impossible that ecological knowledge applied in
advance could prevent some erosion and drainage problems and
save some of the destruction of natural vegetation. Certainly road-
side ecology is worth considering both practically and aesthetic-
ally.

PLANT INDICATORS

Elsewhere we have emphasized that plant communities give a
better indication of the nature of environment than we can obtain
by measurements of individual factors. The character and make-
up of vegetation is an expression of the integrated effects of all
factors operating in a habitat. When the relationships involved are
well known, the vegetation becomes an indicator that can be in-
terpreted or, in some instances, read like an instrument.

The practical use of plants as indicators is nothing new, for
Pliny135 wrote of selecting soil for wheatland by the natural vege-
tation it supported. More recently, in the settling of North Amer-
ica the pioneers used the principle widely in selecting their lands

APPLIED ECOLOGY 351

for agricultural purposes. With increasing knowledge, their selec-
tions became more effective as is indicated today by lands that
have been abandoned and that have remained so. In any agricul-
tural region, an experienced farmer knows the characteristics of
soils and habitats supporting local peculiarities of vegetation, or
often only a single indicator species.

Such practices and beliefs are usually the result of trial and
error experiences, as well they must be, until the responses of a
crop plant are tested under the conditions indicated by native
vegetation. The knowledge has often been acquired after costly
experience. If the requirements of an introduced plant are known
and the characteristics of the habitats of native species are studied,
the guessing may be reduced. Selection of native species as indica-
tors of local conditions and fitting the ecological requirements of
appropriate cultivated plants to these conditions involves ecologi-
cal methods and thinking. Actually this is not easily accomplished,
because of our still limited knowledge of the ecology of both na-
tive and cultivated plants. It suggests the possibilities of the indica-
tor method, however, in an applied field.

The scope of possible uses of indicators involves much of the
entire field of ecology, which necessarily limits the discussion
here. Clements'57 exhaustive treatment explores most of the possi-
bilities of their application, and many of these we have considered
in other connections. Consequently, only certain practical aspects,
in which they have been successfully applied or might be further
expanded will be discussed. The available source material has been
brought together, and a review is available on the modern status
of the concept and its application.212

It may sometimes be difficult to recognize or select indicator
species. Those with restricted distributions and those tolerating
only narrow ranges of habitat conditions should be most useful.
Such plants should show responses to. minor habitat differences.
Thus it follows that similar local conditions in different climatic
areas would probably support different indicators. Also the same
species might not always be indicative of the same things through-
out its range. Differences in geological or cultural history might
make it necessary to interpret the significance of an indicator since
it need not always be the same. It is rather generally agreed that

352 THE STUDY OF PLANT COMMUNITIES * Chapter XII

a group of species or a whole community is more reliable as an
indicator than a single species and that dominants, especially of
the climax,57 or at least characteristic species34 are more useful in-
dicators than lesser species. Above all, application of the method
cannot be successful without judgment, good sense, and interpre-
tation in terms of each situation.

Agricultural Indicators.— That crop centers and types of agri-
culture are correlated with climate and climax vegetation is obvi-
ous. The agricultural areas of North America follow a pattern
very similar to that of a map of natural vegetation.229 The north-
eastern conifer region suggests general agriculture at the lower
altitudes and latitudes where the land is level and soil is deep. In
the transition from boreal to deciduous forest, white pine-red
pine-jack pine forests are on sandy soils, which are, in general,
undesirable for agriculture, while the northern hardwoods-hem-
lock forest indicates the best soils for cultivation. The range of the
deciduous forest formation marks the best agricultural region of
the east with the greatest diversity of crops. Away from the south-
ern Appalachian and Ohio Valley center, as the associations be-
come less complex and oak and hickory become relatively more
important, so also does agriculture become more specialized.

On the prairie, both tall and mixed grasses indicate fertile and
productive land for cereals, hay, and fodder. Likewise, the natural
grass cover provides valuable grazing facilities. The short grass
area indicates productive soil whose cultivated crops are limited
by moisture. The most favorable sections can be dry-farmed, but
otherwise irrigation is necessary for cultivation. As a result, the
land is most widely used for grazing.

Vegetation indicating general land use has been given more at-
tention in the western United States than elsewhere.230 Subalpine
vegetation indicates a growing season too short for cultivated
crops, steep slopes, and poor agricultural soil. The montane zone
also has a short season with cool weather but permits some culti-
vation if the land is not too rough. Pinon-juniper in the woodland
zone indicates productive soil if irrigation is possible, but chap-
arral indicates inferior agricultural land under almost any circum-
stances.

Plant indicators of land use in the arid regions of the West are

APPLIED ECOLOGY

353

rather well known because of several intensive studies in different
areas. Irrigation is necessary everywhere except on the best soils
in the sagebrush areas of the northern portion of the Great Basin.
Elsewhere, in addition to the need for irrigation, native species in-
dicate other necessities or precautions.228 The tabulation on page
354 although specifically applicable only to the Sonoran Desert
region of Arizona and southeastern California, illustrates the prin-
ciples involved.

FlG. 188. These productive fields and orchards in Hurrican Valley, Utah,
irrigated from the big ditch at left, are bordered on all sides by sagebrush
desert. Knowledge of natural vegetation and soil gained from such projects
makes possible confident statements of probable success or failure when
others are to be established— U. S. Forest Service.

These generalizations indicate how natural vegetation may be
useful in determining regional land use. It is the details of local
conditions as indicated by native species that need more study.
If the equivalent cultivated and native species were known for
different soils, sites, and exposures, it would be possible to state
with confidence which fields should be cultivated and which
should be put to pasture or wood lot, as well as which crops
should be grown in a particular field. The more complete such
knowledge is, the more effectively land can be used, and the more
certainly land values can be fixed for sale and taxation.

354 the study OF plant communities • Chapter XII

TABLE 11— Potentialities of Lands for Crop Production as Indicated by the
Principal Plant Communities of the Southwestern Desert (after Sampson212).

Vegetation

Creosote bush

Desert sage

Mesquite and
chamiso

Chamiso

Mesquite thicket.

Seep weed

Saltbush and
arrowweed .
Pickleweed . . .

Saltgrass

Yucca-cactus .

Giant cactus-
paloverde . . .

Predominant species

Larrea divaricata

Atriplex polycarpa

Prosopis glandulosa
Atriplex canescens

Atriplex canescens .
Prosopis glandulosa

Dondia intermedia .

Probable success
under irrigation

Atriplex lentiformis

Pluchea sericea

Allenrolfea occidentalis

Distichlis stricta

Yucca mohavensis
Ferocactus acanthodes
Oppuntia bigelovii . . .

Carnegia gigantea
Cercidium torreyanum

Successful where na-
tive cover is luxuri-
ant; of doubtful suc-
cess on lands of rock
outcrop or with rock
layers or hardpan
Successful where
native cover is lux-
uriant; of low value
on hardpan soil
Partly successful;
special crops on
level tracts
Successful
Successful when
salts leached out
Not successful ;
much abandoned
farm land on this
cover. Successful
when salts leached
out

Successful when
drained

Successful when
drained and leached
Successful when
drained and leached
Partly successful;
land usually too
steep or soil too
rocky

Successful when
drained

APPLIED ECOLOGY

355

Land evaluation on an ecological basis has been made use of at
various times, and a simple illustration will serve to indicate the
possibilities. Not long ago the construction of dams for water
control in the upper Mississippi River necessitated legal action to
fix the value of much lowland that would be flooded when the
project was completed. One of the basic questions involved the
establishment of criteria for determining which acreages were
cultivatable and which were not. It was possible to show by means
of the natural vegetation, regardless of whether the land had or
had not been cultivated, which areas were only rarely flooded
and, therefore, desirable agriculturally, which flooded frequently,
and which were always too wet for cultivation. Once this was
worked out it could be applied generally throughout the area.
The information was used effectively for establishing equitable
land values in several court proceedings.

Range and Pasture Indicators.— The use of plants as indicators
is basic to range management.248 A knowledge of the important
indicator plants and the application of their meaning to handling
of grazing land has become fundamental to successful manage-
ment. Plant indicators are used to judge the condition of the range
and particularly to recognize signs of deterioration or improve-

FlG. 189. Death of shrubs and a browse line in a pasture as indicators of
too heavy grazing by cattle— U. S. Forest Service.

356 THE STUDY OF PLANT COMMUNITIES * Chapter XII

ment under certain usages. They are used to determine the kind,
degree, and time of grazing, and for determining the grazing
capacity of a range. When the plants present are considered in
conjunction with soil conditions and the climax, the previous use
of the range can be interpreted and its potential usefulness under
proper management can be predicted.

FlG. 190. Winter range (Atriplex nuttallii) in Colorado, so badly over-
grazed that there is practically no vegetation left and gullying is serious on
all the slopes. Such depletion is obvious to anyone, but recognition of the
onset of these conditions should be possible for those who know the indica-
tors.— U. S. Forest Service.

Misuse of range lands is obvious in late stages, but it is difficult
to recognize when it first begins and should be corrected. Among
the indicators that must be watched for are thinning of cover and
a lowered vitality of the principal species, replacement of good
forage plants by inferior ones, close grazing of species that ordi-

APPLIED ECOLOGY 357

narily would not be preferred, and, with this, accelerated ero-
sion.249 It is also highly desirable that the slow successional changes
in species composition resulting from grazing under a certain
system be recognized. Usually if these are in the direction of
climax, they are advantageous. If they show an increase of forbs
or of unpalatable species, management practices must be corrected
before the trend becomes serious.

In each grazing region, the significant indicators must be known
and interpreted. Often selected species can be used and checked
upon to simplify evaluations. Likewise, restricted areas, selected
on the basis of experience, may be used for observation as repre-
sentative of the general conditions on a range as a whole.

Range management is obviously applied ecology in which indi-
cators play an important part. The more completely the ecology
of the species and communities is known under grazing condi-
tions, the more readily their responses can be interpreted and the
more effective management practices can be.

Forest Site Indicators.— In forestry, as in agriculture, the indica-
tor significance of one group of plants must be interpreted and
applied to an entirely different group of plants. Since forest indi-
cators are commonly herbs or shrubs, there is often some diffi-
culty in translating their meaning to apply to trees. In the broad-
est sense, forest indicators are site indicators, but rarely do they
suggest more than a portion of the several factors that contribute
to site. Physical or chemical characteristics of soil, moisture rela-
tionships, aeration, or erosion may be indicated by some species.
With these and others the probable development of a particular
stand can be interpreted. Still others may indicate the past history
of vegetation on the site or the probable successional trend to be
anticipated in the future.

It is fundamental to indicator interpretation that the succes-
sional trends of a region be thoroughly understood for every type
of habitat. Only when an indicator is considered in relation to the
stage of succession concerned can its meaning be at all clear.

The use of subordinate or dependent species as indicators of
site quality has been attempted under various conditions since
Cajander51 set up such a system for classifying forest types in
Finland. This system assumes that, since communities of similar

358 THE STUDY OF PLANT COMMUNITIES * Chapter XII

structure occupy similar sites, it is possible to judge a site and the
nature of the dominants from the ground cover alone. Thus recog-
nition of the herbs, mosses, and lichens on the forest floor with an
estimate of their relative proportions might suffice for evaluation
of the stand and the quality of the site on which it grows.

Perhaps the most comprehensive attempt to apply the method
in North America was made in the Adirondack Mountain area.121
Elsewhere smaller areas with fewer communities have been studied.
Although special phases of the method have proved useful in cer-
tain situations, the method as a whole has found limited applica-
tion. Although herbs undoubtedly affect the dominants by modi-
fying soil structure and water relations, and likewise through
competition with seedlings of dominant species, there are argu-
ments against the validity of information based on herbs alone,
particularly since they derive water and nutrients from different
soil horizons than do the dominant trees. It is, therefore, suggested
that all the lesser woody vegetation should also be included. There
is evidence that the same herbaceous species predominate on more
than one soil type, and, therefore, their significance is questioned.
Often the indicator types are of limited extent, and several may be
present within a single stand. Interpretation, then, becomes diffi-
cult in terms of management. Undoubtedly, the foresters' not
uncommon lack of familiarity with lesser vegetation and frequent
inclination to ignore it entirely have been factors in limiting the
testing and application of the method in American forests.

Because the subordinate vegetation changes after lumbering or
fire and because height of trees, the commonest criterion of site,
cannot then be known, it is desirable that some relatively simple
means of evaluation of site be available that can be applied at any
time. T S. Coile65 has approached this problem through physical
measurement of the soil, as others have attempted before, and,
after extensive investigation, has found that the site index can be
accurately determined if only the depth of the A horizon and the
soil type are known. Using the xylene equivalent (determined like
moisture equivalent) of the B horizon (known for the soil type)
and the depth of the A horizon, a positive statement of site quality
can be made whether the land is in forest, cultivated, or abandoned
and regardless of slope or exposure. This would seem to be the

APPLIED ECOLOGY 359

most promising approach to recognition of site quality. Once
these two factors are known for the soils of an area, they can be
recorded like a soils map, which then becomes a map of site index
to be interpreted for management purposes.

Innumerable indicators, other than site indicators, are used in
forestry. Relicts are particularly useful, and successional indi-
cators are applied regularly. Special instances have been suggested
elsewhere. It is appropriate to emphasize that indicator applica-
tions are invariably successful when the ecology of the region and
the species is known.

HUMAN ECOLOGY

Perhaps this final section seems out of place in a textbook
intended as an introduction to plant ecology. Undoubtedly, its
subject should not be looked upon as an application of ecology in
the sense of the preceding paragraphs of this chapter. The intent
is to emphasize that all organisms are related to their environments
and, consequently, to each other and that, therefore, they will be
best undestood when studied from an ecological point of view.

Considering the youthfulness of the science of ecology, it has
contributed much to our understanding of plants and animals at
the same time that its methods have won approval and even adop-
tion in other fields to the benefit of all. Although we still have
plant ecologists and animal ecologists, and probably will continue
to have such specialists, there has been a steady increase in the
appreciation of interrelationships among plants and animals.61

Furthermore, there is a growing realization that man is like-
wise subject to ecological laws. This is completely reasonable
since man, like other organisms, is basically dependent upon his
environment and is likewise a factor in that environment. With
man's increasing dominance, it is desirable that these relationships
be better understood. How better can one approach that under-
standing than through studies of the structure of the communities
in which man dominates, their origins and successional develop-
ment, and the controlling factors involved. This is human ecology.

This is not a new idea, but it has not been widely recognized
or accepted. There is much evidence that it is gaining recognition.
There is an increasing appreciation of the concepts and values of
ecology among the public in general as evidenced by the not

360 THE STUDY OF PLANT COMMUNITIES * Chapter XII

uncommon use of the term in popular magazines and even occa-
sionally in newspapers. This represents one phase of progress.
The other is indicated by the use of the term and ecological
methods by scholars and investigators in fields ordinarily not
thought of as ecological. Anthropologists have undoubtedly led
the way in adapting ecological methods to their problems and
have, consequently, influenced others to try similar applications
in different fields. Although the social ecology of animals has been
given much attention,0 there have been only a few advocates of
ecological methods in the analysis of man's social behavior.2 How-
ever, human ecology is gaining increasing recognition among
sociologists under the pioneering influence of a few of their
number177, 178 who have thought in terms of social ecology for
many years. As a part of the interpretation of man's activities
and responses, it follows that certain phases of psychological
action must likewise be given consideration in human ecology.
Also, if human communities are to be studied as a whole, econom-
ics, too, becomes susceptible to ecological interpretation. These
things make it apparent that human ecology is a comprehensive
subject but one with promise of substantial returns for its study.

Some ideas of human ecology as expressed by a sociologist177
seem particularly pertinent here. The scope of human ecology is
so great that it must have a synoptic view of plant, animal, and
human communities since all are interrelated and governed by the
same principles involved in competition, symbiosis, succession,
balance, and optimal population. Approached in this fashion, the
laws, processes, and structure of human population are seen to be
subservient to the more comprehensive laws of ecology since the
latter are the determiners of regional economic and social types.
When the arrangement and spatial adaptations of populations are
considered, such ecological processes as aggregation, mobility,
specialization, distance, and succession are excellent bases of
evaluation. They permit the establishment of ecological indices
for the measurement of types and trends of social mobility, dis-
stance, dominance, and change.

Finally, let us return to a phase of the discussion that has been
touched upon earlier in several connections. No science can be
completely justified for itself alone since science is supported by

APPLIED ECOLOGY 361

society. It is hoped that, in this last chapter, enough practical
aspects of ecology have been suggested to show its wide appli-
cability. Furthermore, the aim has been to show that its application
is necessary if man is to continue to enjoy the full benefits of his
environment upon which he is dependent, in which he is a factor,
and over which he is a dominant. We have suggested that people
with a wide variety of interests have concerned themselves with
the general subject of human ecology. Among plant ecologists,
Dr. Paul B. Sears is outstanding for his efforts in behalf of applied
ecology and; particularly, human ecology. As a conclusion to this
section it is, therefore, entirely proper that we quote one of his
chapter headings from "Life and Environment"220 which reads,
"The social function of ecology is to provide a scientific basis
whereby man may shape the environment and his relations to it,
as he expresses himself in and through his culture patterns!'

GENERAL REFERENCES

C. C. ADAMS. General Ecology and Human Ecology.
H. H. Bennett. Soil Conservation.

F. E. CLEMENTS. Plant Indicators : The Relation of Plant Communities to

Processes and Practice.
I. N. GABRIELSON. Wildlife Conservation.
E. H. Graham. Natural Principles of Land Use.
C. E. KELLOGG. The Soils That Support Us.
K. H. W KLAGES. Ecological Crop Geography.
R B. SEARS. Life and Environment.
H. L. SHANTZ. Natural Vegetation as an Indicator of the Capabilities of Land

for Crop Production in the Great Plains Area.
L. A. Stoddart and A. D. Smith. Range Management.
J. W TOUMEY and C. F. Korstian. Foundations of Silviculture upon an

Ecological Basis.

References Cited

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2. Adams, C. C. General ecology and hu-

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3. Aikman, J. M. Native vegetation of the

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6. Allee, W. C Animal Aggregations. A

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8. Anderson, L. E. The distribution of

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9. Anderson, P. J., and Rankin, W. H.

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10. Anderson, R. M. Effect of the intro-

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15. Bbard, J. S. Climax vegetation in trop-

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20. Billings, W. D. The structure and de-

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22. .Vegetation and plant growth as

affected by chemically altered rocks in
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23. , and Drew, W. B. Bark factors af-
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24. Blumenstock, D. I., and Thorn-

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25. Booth, W. E. Tripod method of making

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26. Bouyoucos, G.J. Making mechanical

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Soil Sci., 25:473-480, 1928.
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'. 1

Index

Page numbers in bold face type indicate illustrative material.

Abelia, 142

affected by length of day, 142
Abies a?nabilis, 276

balsamea, 63, 240, 241, 243, 245, 248

concolor, 70, 264, 272, 274

fraseri, 244, 245

grandis, 277, 279

lasiocarpa, 244, 261, 276

magnified, 70, 270, 271, 274

nobilis, 276
Abstract communities, 69
Abundance, 56-58
Abundance scale, 58
Acer glabrwn, 70

rubrum, 63, 249

saccharum, 63, 246, 247, 248, 249,
253, 255
Acid and alkaline soils, 178-179

see Alkalinity, pH
Adaptation and survival, 30
Adaptations, aeration

aquatic plants, 175-176, 217-218

emergent plants, 175-176

lacunar tissue, 175-176

pneumatophores, 176

submerged leaves, 176
Ad eno stoma jasciculatum, 139
Aeration, 174

and leaf structure, 138-139

decreases with depth (soil), 174

toxicity, 182
Aes cuius califomica, 275

octandra, 248, 249
Agave, 290
Agricultural indicators, 352-355

crop centers and natural vegeta-
tion, 352

land evaluation, 355

land use, 352-353-354
Agriculture, 326-337

crop ecology, 326-327

land management, 329-332-333

land use, 328-329

pasture problems, 333-336

pests, 336-337
Agropyron, 268

repens, 292

smithii, 297

spicatum, 297
Agrostis alba, 292
Ao horizon, 154
Air capacity of soil, 175
Alkalinity, of soil, 178-179

calciphiles, 183

causes, 183

pH, 178-179

plant relationships, 183
Alluvial soils, 149-150

texture of, 150
Alnus incana, 242
Alpine soil, 145
Alpine tundra, 236, 239-240

location, altitudes, 239
Alpine vegetation, 145, 236

Krumviholz, 145
Altitudinal zones, 124, 133

in Utah, 125
Amelanchier spp., 269
Ammonification, 196-197
Andromeda polifolia, 239, 242
Andropogon, 218, 219, 221

furcatus, 292, 295

littoral is, 51, 52

saccharoides, 293

scoparius, 292, 295

tener, 293

ternarius, 293
Anemometers, cup and Biram, 99
Animals •

as dependents, 26

as factors

dissemination, 198-199
grazing and browsing, 201-202
in soil, 200
man, 202-210

371

372

THE STUDY OF PLANT COMMUNITIES

Animals, as factors— Continued
pollination, 198
soil organisms, 200
as influents, 26
Animals, of soil, 200-201
macrofauna, 201
microfauna, 200
Applied ecology, 315-361
agriculture, 326-337
and secondary succession, 216
conservation, 337-347
forestry, 316-321
human ecology, 359-361
landscaping, 347-350
plant indicators, 350-359
range management, 321-326
Aquatic plants
aeration, 175-176
characteristics, 217
emergent, 217-218
floating leaved, 217-218
lacunar tissue, 175
Aralia nudicaulis, 241
Arbutus menziesii, 281
Arctic tundra, 238-239

climax, 239
Arctostaphylos spp., 274, 275
glauca, 282
tomentosa, 139, 282
Aristida longiseta, 297

stricta, 253, 256
Arte?nisia spp., 268, 286
tridentata, 284, 285
spine sc ens, 285
Artificial forest types, 317-319-320
Asclepias mexicana, 199
Aspect dominance, 67, 68

in grassland, 297-298
Association, 225-226

individual, 45
Aster, 221

acuminatus, 241
Asymmetric growth and wind, 101-

102-103
Atmometers, 85-86

description and operation, 85-86
indicators of light, 85
Atmosphere
capacity to hold moisture, 78
gaseous content, 75-76
of the soil, 174-177
variations in composition, 76
water content, 77
Atmospheric moisture, condensation

causes, 87

clouds, 87-88

cooling of air masses, 87

fog, 87, 88, 89

precipitation, 88-95
Atmospheric moisture

and evaporation, 78, 79

and vegetation, 95-97

dew point, 86

measurements, 81-82

plant distribution, 96

precipitation of, 77-78

relationship to temperature, 77

saturation, 77

terminology of, 78-79
Atmospheric pressure, 97-98

relation to wind, 97-98

varying with temperature, 97
Atriplex spp., 286

confertifolia, 285

nuttallii, 356
Autecology, 17

and physiology, 17

in the field, 20
Auxins, 135-137

and differential growth, 137

formation of, 135-136

relation to size, shape, 136
Available water, 170-171

and root growth, 68, 164

capillary rise, 164

degrees of availability, 171

in different soils, 171

soil solution, 171

soil temperature, 171

Bacteria, soil
nitrates, 196-198
nitrogen fixing, 196
nodule bacteria, 195-196
succession of bacteria, 197

Balance of population, 207-209
biological control, 208
destruction of predators, 208
introduced species, 207-209

Basal area, 59
determination, 62
in phytographs, 62, 63
relation to dominance, 62

Base exchange, 179-182

Batodendron arboreum, 259

Beech-maple association, 249-250

Betula lutea, 63, 245, 250

INDEX

373

Betula latea— Continued

nigra, 255

papyrifera, 63, 240, 245, 269
Bidens frondosa, 199
Biological factors, 187-210

animals, 198-210

competition, 188-190

plants, 187-199
Biological balance, disturbance by

man, 209-210
Bisects, 50, 54
Black Hills vegetation, 269
Bluffs

moisture-temperature and expo-
sure, 31
Bog

development, 216

drainage, cultivation, 209

floating type, 216

forest, 216, 311

succession, 216
Boreal forest formation, 240-245

Appalachian extension, 244-245

climax, 240-241

range and climate, 240

successions, 241-244

transitions, 243-244
Bouteloua, 268

curtipendula, 292

gracilis, 292, 293, 295, 296

hirsuta, 292, 293, 295
Broad sclerophyll formation, 280-283

broad sclerophyll forest, 280, 281

chaparral, 280, 281, 282

fires, 282-283

ranges, distribution, climate, 280-
281
Bromus tectorum, 227, 297
Browse line, 201, 208-209, 355
Buffalo, as a factor, 202
Bulbilis dactyloides, 293, 295, 296
Buried forest, 112, 114

Calamovilfa longifolia, 293

Calcification (soil), 156-157

Calciphiles, 183-184

Calcium compounds, and soil, 183
tolerance to, 183

Calliergon giganteum, 303

Capillary capacity, 168

Carbon dioxide
content of air, 75
relation to soil depth, 174

Carex capillaris, 239

nardina, 239

rupestris, 239
Carnegiea gigantea, 288
Carnivorous plants, 257
Gary a alba, 254

cordiformis, 254

laciniosa, 254

ovata, 254
Cassiope tetragona, 239
Castanea dentata, 251-252
Castajiopsis chrysophylla, 140, 281
Ceanothns, 268, 274, 275

cuneatus, 282
Celtis spp., 255, 269
Cejichrus pauciflorus, 199
Cercidium microphyllwn, 288
Cercis canade?isis, 25
Cerococarpus, 268

betidaeformis, 282

ledifolius, 269

parviflorus, 269
Chamaecy paris lawsoniana, 279

thyoides, 257, 258
Chamaedaphne calyculata, 242
Chaparral, 28, 275, 280, 281-282-283

and fire, 282-283

growth form, 28

leaf structure, 139
Characteristic species, 72

indicator significance, 73
Chestnut blight, 190, 252
Chiogenes hispidula, 241
Chlorophyll and light, 135-136
Chrysopsis breweri, 272
Chrysothamnus piiberulus, 286
Circle of illumination, 116, 117, 118
Classification of communities

basis of life, form, 20

static and dynamic viewpoints, 1 7
Classification of vegetation tvpes

associations, 225

faciations, 225

formations, 225

lociations, 225
Cladonia leporina, 218
Climate

and climax, 160, 224

and soils, 157-161

and vegetation, 15, 160

kinds of plants, 28
Climatic factors

air, 71-115

control of growth form, 31-32

374

THE STUDY OF PLANT COMMUNITIES

Climatic factors— Continued
insolation, 116-118
precipitation, 88-97
radiant energy, 116-143
temperature, 118-128
Climax

an indicator of climate, 224
basic concept, 226
characteristics, 223-224
monoclimax interpretation, 226-229
polyclimax interpretation, 226-229
present distribution of, 234-235-

299
relation to climate, 223-224
relationships of successional trends,

223-224
stability, 224
types, 226-229

uniformity and variations, 224-226
variations related to time, 224
Climax communities
present distribution, 236-299
shifts with time, 301-314
Climax, distribution of, 234, 235-299

controlling factors, 234
Climax formations of North America
listed, 237
map, 235
Climax regions, 225
formations listed, 225
uniformity of life form, 224-225
Climax regions of North America

by formations, 236-299
Climax, study of

criteria for recognition, 230
procedure in local study, 230-233
sampling, 232-233
use of quantitative data, 231-232-
233
Climax, types of
disclimax, 227
edaphic, 226
physiographic, 226
postclimax and preclimax, 227-229
subclimax, 226
Climaxes of past, reconstruction, 301-
314
dendrochronology, 308-310
paleo-ecology, 301-304
pollen analysis, 304-307
relict method, 310-314
Climographs, 98
Clintonia borealis, 241

Clouds
causes, 87-88
classification, 88
effect on temperature, 125
source of precipitation, 88
Coefficient of community, 74
Cold air drainage, 98, 124
Cold front (air masses), 87
Coleogyne ramosissima, 286
Colloids
and exchangeable bases, 180
and soil characteristics, 153
and soil water, 162
Colluvial soils, 150

talus, 151
Community
abstract, 21, 69-74
analysis a necessity, 34
basic vegetational unit, 21
classification by life form, 20
concrete, 21
definition, 21
description justified, 33
first recognized as basis of study,

16
fixing the concept of, 33-34
illustrated, 18, 19
its nature, 21
recognition, 21
size, 21

synthetic analysis, 69-74
Communities (layer or strata)
of the forest floor, 23, 24
synusia, 25
Community disturbance

drainage, fire, irrigation, 203
Community dynamics, 211-314
methods of study, 229-233
plant succession, 211-233
present distribution of climaxes,

234-299
shifts of climaxes with time, 300-
314
Community structure
Quantitative characters, 56-63
cover and space, 61
density, 57
frequency, 57
numbers of individuals, 56
Qualitative characters, 64-69
dispersion, 64
periodicity, 65
sociabilitv, 64
stratification, 65

INDEX

375

Community structure, qualitative

characters— Continued
vitality, 64
Compass plants, 137
Competition, 21-24, 188-190

and dependent species, 25

and soil moisture, 30

causes of, 22

direct (physical), 188

intensity of, 22

introduction of new species, 189-
190

through physiological require-
ments, 188

tree seedlings, 25
Conopholis americana, 27
Constance, 71-72

diagram, 71, 72
Conservation, 337-347

soil, 338-341

water supply, 341-344

wildlife, 345-347
Coptis trifolia, 241
Cormts canadensis, 241

florida, 254
vernal aspect, 25
Cover, 61-62

and temperature, 125-127

by strata, 62

classes, 62, 66

estimation, 61-62

in grassland studies, 62

measurement, 61-62

square foot density, 62
Coverage classes, 66
Cover-stratification diagrams, 66
Cowa?iia, 268
Crop ecology, 326-327
Cuscuta, 191
Cy penis, 175
Cypress swamp, 31, 176
Cyrilla racemiflora, 257

Dalea, 288

Dasylirion longisshnum, 290
Death Valley, 287-288
Deciduous forest (beech-maple), 18
Deciduous forest formation, 245-259
beech-maple association, 18, 249-

250
hemlock-hardwoods association,

250-251
maple-basswood association, 249-
250

mixed mesophytic association, 245,
247-249

oak-chestnut association, 251-252

oak-hickory association, 252-255

range, climate, topography, 245-
256
Decomposition and available nitro-
gen, 197
Deer, 26, 201, 208-209
Dendrochronology, 308-310

applications, 308

correlations with climate, 309-310

methods, 308, 309

sunspot activity, 310
Density, 58, 59

in phytographs, 63, 231

applied in succession, 231-232
Dependence

animals, 26

community, 23

Conopholis americana, 27

epiphytes, 26

kinds of organisms, 26

Monotropa imiflora, 27

parasites, 26

saprohytes, 26
Desert formations, 283-289

areas, 283

Desert Scrub, 286, 287-289

extent, climates, conditions, 283

Sagebrush, 284, 285-286
Desert Scrub formation, 286, 287-289

Chihuahua desert, 289

Mojave desert, 287-288

Sonoran desert, 288, 289
Dew point, 78
Dionaea muscipida, 257
Disclimax, 226

Rromus tectorum, 227

Opuntia, 227
Dispersion, 64
Disseminules, 199

animal transported, 198-200

transporting devices, 199-200

wind transported, 108-109
Distichlis spicata, 185
Distribution of vegetation

and temperature zones, 15, 324

causes, 16, 324-325

correlation with single factors, 15,
324
Dominance, 23

aspect, 65

criteria of, 25

376

THE STUDY OF PLANT COMMUNITIES

Dominance— Continued

relation to basal area, 61

relation to cover, 61

seasonal, 65
Dormancy and photoperiodism, 142-

143
Drainage, artificial, 209
Dryas octopetala, 239
Dryopteris dilatata, 241
Dunes (see Sand dunes)
Dust storm, 109

Earthworms, 200
Ecological training, 14
Ecology

applied, 315-361

approaches to the subject, 16

breadth of the field, 14

definition, 11

human, 13, 359-361

objectives, 12, 13

practical considerations, 315-361

scope, 13

static and dynamic viewpoints, 17

subject matter, 11, 13
Edaphic factors, 144-174
Eichornia, 206-207
Elymus condensatus, 297
Elyna bellardii, 239
Empetrum nigrum, 239
Environment

a complex of factors, 16, 75

and life, 12

and physiological processes, 12

climatic factors, 75-143

components, 13

defined, 13

factors, 13
Ephedra spp., 287
Epilobium latifolium, 239
Epiphytes, 26, 193

latitudinal distribution, 193

Spanish moss, 28, 193

specificity, 193

throughout plant kingdom, 193
Equinoxes, 116
Eriophorum spp., 238
Erodium cicutarium, 199
Erosion, 327

control, 333, 334-335
Euphorbia, ipecaciianhae, 257

polygonifolia, 52
Eurotia lanata, 286

Evaporating power of the air, 85
Evaporation

and transpiration, 82-83
measurement
atmometer, 85, 86
evaporimeters, 85
open tank method, 85
precipitation ratio, 96, 97
Evernia vulpina, 272
Exchangeable bases, 179-182
Exclosures, 42, 312
types and uses, 43
Exclusives (fidelity), 72
Exposure and insolation, 133

Faciation, 225

Factors, of the environment, 13

air, 76-113

and plant distribution, 15

biological, 187-210

climatic, 75-143

exchangeable bases, 179-182

insolation, 116-143

organisms, 187-210

physiographic, 144-187

soil, 144-161

soil acidity, 178-179

soil atmosphere, 174-177

soil water, 161-174

temperature, 118-127

topography, 185-187

wind, 97-115
Fagus grandifolia, 246, 248, 249-250,

253
Fairy rings, 182
Fallugia paradoxa, 269
Festuca idahoensis, 297
Fidelity, 72-73

and constance, 73

characteristic species, 72-73

classes, 72
Field capacity, soil, 168
Field margin, plantings, 331
Fimbristylis casta?iea, 52
Fire

and pine savannah, 254, 256

as a factor, 215, 266-267

controlled burning, 205

effects, 203-204-205, 226-227, 282-
283
Fish ponds, 329
Fixation of nitrogen, 196-197
Flourensia, 297

INDEX

377

Fog, 87

causes, 87

coastal and inland, 87, 88, 89

relation to vegetation, 87
Food chains, 12
Foothills forest

Rockies, 267-269

Sierra Nevada, 274, 275-276
Forbs, 298

Forest site indicators, 357-359
Forest types, artificial, 317-319-320
Formations, 225

criteria for recognition, 229
Foiiquieria splendens, 289
Franseria dumosa, 288
Fraxinus spp., 255

americana, 63, 250

caroliniana, 258

profunda, 258
Frequency, 58, 59

and size of quadrats, 59, 60

classes, 59

classes and homogeneity, 61

diagrams, 61, 71

in oak-hickory forest, 231-232

used in phytographs, 63, 231

Kenoyer's normal, 61

meaning of classes, 60

Raunkiaer's, law of, 60

Raunkiaer's normal, 61
Frost injury

abelia, 142

and hardening, 142

desiccation, 118
Frost penetration of soil, 127

under snow, litter, 127
Fungi, as factors, 26

of the soil, 194-198

parasites, saprophytes, 194-198

Gaultheria shallon, 278

Gay ophy turn ramosiss'nnum, 272

Germination

and aeration, 174

and temperature, 127

growth inhibiting substances, 182
Glacial soils, 151-152
Glaze, 90

damage, 90, 91
Gordonia lasianthus, 257
Grassland formation, 290-298

aspect dominance, 297, 298

extent, transitions, general climate,

290-291
mixed grass prairie, 293, 294-296
other grassland climax, 297
short grass plains, 295, 296
tall grass prairie, 291-292-294

Grassland precipitation, 95

Grassy balds, 18

Grayia spinosa, 286

Great Salt Lake, saline vegetation,
185

Gregariousness, 64
Grimmia laevigata, 218
Growth form

indicator of climate, 28

controlled by climate, 28-29, 31-32

Habitat, 30

hydric, 216-217

local variations, 30-31

mesic, hydric, xeric, 216-223
Halophytes, 184

xeromorphism, 184
Hammock vegetation, 256-258
Hardening and frost injury, 142

Abelia, 142
Hechtia, 290
Hemlock-hardwoods association,

250-251
Heteromeles arbutifolia, 282
Heterotheca subaxillaris, 51, 52
Hydrogen ion concentration, 178-
179

and acidity, 178

pH, 178-179
History of plant ecology, 15
Holoparasites, 191
Homogeneity of vegetation, 61, 64
Hudsonia, 252
Human ecology, 13, 359-361
"Humidity (see also Relative Hu-
midity)

absolute, 78

relative, 78
Humus, 154

mull and mor, 154
Hydrarch succession, 215, 216, 217
Hydrophytes, 137-139, 215-217
Hygrometer, 81, 82
Hygroscopic coefficient, 167
Hygrothermograph, 82
Hyoscyamus niger, 141
Hyperdispersion, 64

378

THE STUDY OF PLANT COMMUNITIES

Hypodispersion, 64
Hypnum crista-castrensis, 23

Ilex glabra, 257

vomit oria, 259
Indifferents (fidelity), 72
Infiltration, 163

on forested and bare land, 94
Influents (animals), 26
Inhibition of growth, 182-183
crop rotation, 182
decomposition products, 182
experimental evidence, 182
fairy rings, 182
seedlings, 182
toxic excretions, 182
Insolation, 116-118
exposure, 133
equinoxes, 116
greatest total, 117
heat, 116

maximum effectiveness, 124
position of the earth, 117
seasonal, 116-117
solstices, 116-117
variations

absorption, 116
angle of incidence, 116
daily, 116, 117
latitudinal, 116, 117
seasonal, 116-117
Interglacial plant remains, 302-303
Introduced species, 207-209, 318
effects of, 206, 207-208
Elodea, 206
gypsy moth, 207
mongoose, 208
muskrat, 206-207
prickly pear, 207
rabbits, 207
sparrows, starlings, 206
water hyacinth, 206-207
Irrigation, 353

Jatropha stimulosa, 257

J uncus, 175

Juniperus cembroides, 267

monosperma, 267

occidentalis, 267, 273

pachyphloea, 267

scopulorum, 267-268

utahensis, 267

Kochia vestita, 286
Koeleria cristata, 292, 293, 295
Krummholz, 101, 102, 145, 263
Knees, of cypress, 176

Lactuca scariola, leaf position, 137
Land management, 329-332, 333
Landscaping, 347-350

ecological relations, 348

natural, 347

road building, 347, 348, 349, 350

Land surveys

reconstruction original vegetation,
53
Land use and ecology, 328-329
fish ponds, 329

hedges and field margins, 329, 331
hillculture, 328
pasture, plowland, forest, 328
stream margins, 330
Larix laricina, 216, 241, 303

occidentalis, 279
Lacunar tissue, 175
Larrea, 297

tridentata, 288, 289
Laterite and laterization, 156
Leaching

and soil acidity, 178
solubility of soil constituents, 145
Leaf arrangement, 136-137
Leaf exposure, 137

profile position, 137
Leaf fall and photoperiod, 143
Leaf structure

affected by water and aeration,

138-139
in mesic habitats, 1 39
in sun and shade, 137-140
Ledum groenlandicum, 242

palustre, 239
Lemaireocereus schottii, 288
Length of day, 141-143
and hardening
Abelia, 142
evergreens, 142
effects on plants, 141-143
Leptilon canadense, 51, 52, 221
Libocedrus decurre?is, 274
Lichens

epiphvtes, 193

in rock succession, 218, 219-220

INDEX

379

Life forms, 18, 19

as basis for classification, 20
Light, 129-143

effect on size, form, 136

chlorophyll production, 139

effect on elongation, 136

flowering, fruiting, reproduction,
140

in forest stands, 130, 132, 133

interception, 132

leaf exposure, 137

leaf orientation, 136-137

leaf structure, 137-139-140

movement and position of chloro-
plasts, 135-136, 139

self pruning, 137

shade tolerance, 133, 134

source of energy, green plants, 129

sun and shade leaves, 137-139
Light and leaf pattern, 135-136

mosaics, rosettes, 135-136
Light and physiological responses

chlorophyll production, 135-136

opening, closing of stomata, 135-
136

photosynthesis, 134-135
Light measurement, 129-132

atmometers, 85, 132

cautions and limitations, 131

photoelectric cell, 129-131

photometer, 130-131

radiometer, 131-132
Light penetration, water, 217-218
Light quality, atmospheric

absorption, 132

diffusion, 132
Light requirements, 132

quality and intensity, 129

vary for species, 129
Light variations, 132-133

biological importance, 132

daily and seasonal, 132

with latitude, 132

with slope, 132, 133
L (litter) layer, 143
Liqiiidambar styraciflua, 254
Limnology, 15
Line transects, 54
Liriodendron tulip if era, 255
Lithocarpus densiflora, 278, 281
Litter

as an insulator, 127

Ao horizon, 154

differential decomposition, 154

L layer, 154

Lociation, 225

Loess, 110, 112-113, 149

Lonicera japonica, 189

in competition, 189
Long day plants, 141

Magnolia acuminata, 248

virginiana, 257
Maianthemum canadense, 241
Man, a dominant, 315-316

responsibilities, 316

must recognize biological laws, 316
Man, a factor, 202-210

a dominant, 203

cities, highways, 203

cultivation, 203

disturbance of biological balance,
209-210

fire, 203-204-205

introduction of species, 206

lumbering, 203

modification of environment, 210
Maple-basswood association, 249-250

day and night temperatures, 127
Maritime forest, 258
Mean temperatures

annual, 122

daily, 121

desert vegetation, 122

maximum and minimum, 122

usefulness of, 122
Mechanical analysis, of soils, 152
Mesophytic leaf structure, 139
Mesophytism, 223
Minimal area, 45
/Mistletoe, 191
Mixed grass prairie, 294-296
Mixed mesophytic forest association,

247-249
Moisture and leaf structure, 138
Moisture equivalent, 169
Mojave desert, 287-288
Monardella odoratissima, 272
Monoclimax versus polvclimax, 226-

229
Monotropa uniflora, 27
Montane forest

Rockies, 263-267

Sierra Nevada, 272, 273, 274
Mor, 154

Mulching and soil water, 165-166
Mull, 154
Mutual relationships

competition, 21

380

THE STUDY OF PLANT COMMUNITIES

Mutual relationships— Continued

energy cycle, 12

food, 12

to environment, 27
Mycorhiza, 26, 194-195

and alkaline conditions, 195

ectotrophic, 194-195

endotrophic, 194-195

of orchids, 195
Myrica calif ornica, 281

cerifera, 52, 257, 259
Myriophyllum, 175

N
Natural resources

can be conserved and used, 316

communities and environments,
315

forests, 316-321

range, 321-326

soil, 326-336

water supplies, 341-344

wildlife, 345
Natural thinning, 24
Neocalliergon integrifolium, 303
Nitrate fixation, 196-197

algae, 197

bacteria, 196-197
Nitrogen in soil

fixation, 196-197

product of decomposition, 197
Nodules

legumes, 196

nitrogen fixation, 196

on leaves. 196

soil fertility, 196
Nolina, 290
Nyssa aquatic a, 258

biflora, 258

sylvatica, 254

Oak-chestnut association, 251-252
Oak-hickory association, 222, 252-
255
fire and swamp subclimaxes, 255-
259
Oak-hickory forest, 222

day and night temperatures, 127
stratification, 25
vernal aspect, 25
Oak-mountain mahogany climax,

268-269
Oenothera hiimifiisa, 51, 52
Ohieya, 288

Opuntia, 268, 288, 297

arborescens, 67

inerviis, 207
Organisms

mutual relationships, 12

reactions on environment, 212-213
Original vegetation, land surveys, 53
Overgrazing, 42, 355, 356
Overstocking, forest, 24
Oxalis montana, 241
Oxydendrum arboreum, 254
Oxyria digyna, 239

Pacific conifer forest, 276, 277, 278-
280

montane zone, 276-280

northern part, 279

range, climate, altitudes, 276

southern part, 279

subalpine zone, 276
Paleo-ecology

fossil evidence, 302

interglacial relicts, 302, 303

methods, 301-302

stratification of peat, 303-304
Panicum virgatwn, 292, 293
Pantograph, in use, 39
Papaver spp., 239
Parasites, 26

beetles, borers, 26

chestnut blight, 190

community structure, 190

dodder (Cuscuta), 191

Dutch elm disease, 190

moths, 26

witches brooms, 191-192
Pasture indicators, 355-357
Pasture problems, 333-336

planting, 334-335

woodlots, 333
Peat bogs, 258-259

development, 216
Peat deposits, 302-303

drained and cultivated, 209

pollen analysis, 304-307
Peat sampler, 304
Pedalfer, 157-158
Pedicularis semibarbata, 272
Pedocal, 156, 157-159
Percolation under litter, 127
Periodicity

aspect dominance, 65

in deciduous forest, 67

leaf fall, 68, 143

INDEX

381

Periodicity— Continued

length of day, 68

of growth, 67-68

seasonal dominance, 65
Persea borbonia, 257

pubescens, 257
pH, 178-179

and microorganisms, 179

and plant responses, 179

determination, 179
Phenology, 65

Phoradendron flavescens, 191
Photometer, 129-131

solarization, 131

uses and limitations, 130-131
Photoperiodism

abscission layers and leaf fall, 68,
143

applied aspects, 141-143

ecological significance, 143

effect on Abelia, 142

failure to become dormant, 143

greenhouse uses, 143

longday and shortday plants, 141

necessary light intensity, 141

seasonal phenomena, 141

vegetative and reproductive activ-
ity, 141
Photosynthesis

and temperature, 128

relations to light, 134-135

Vant Hoff's law, 128
Photosynthetic efficiency

light, 129

species differences, 129
Phytographs, 62, 63

in climax studies, 231

oak-hickory, 231
Phytometers, transpiration, 83
Phytosociology

basic problems, 55

development of, 55

in successional studies, 233

objectives, 55-74
Picea engehnanni, 145, 261

glauca, 240, 241, 243, 261, 269, 303

mariana, 216, 241, 303, 311

rubens, 63, 244

sitchensis, 279
Pifion-juniper climax, 267-268
Pinus albicaulis, 272

aristata, 263, 264

attejinata, 274

balfouriana, 273

banksiana, 241, 242, 248

caribaea, 254

contorta, 70, 262, 265, 271, 272,
274

echi?iata, 252, 255

flexilis, 145, 265, 273

jeffreyi, 274, 276

la??ibertiana, 272, 274

latifolia, 266

leiophylla, 266

mo?iticola, 70, 271, 279

muricata, 274

murrayana, 194
mycorhiza, 194

palustris, 253

ponderosa, 265, 266, 268, 269, 272,
313

var. arizonica, 266
var. scopulorwn, 265

resinosa, 243, 251, 318, 319

rigid a, 252
var. serotina, 257

sabiniana, 275

strobiformis, 264

strobus, 243, 249, 250, 318, 319

taeda, 24, 255, 319
in succession, 222

virginiana, 252, 255
Pioneer plants, 219-220

hydrarch succession, 216-217

xerarch succession, 218
Pirola picta,. 272
Plants as factors

competition, 188-190

epiphytes, 193

parasites, 190-192, 193

soil flora, 197-198

symbioses, 193-194
Plant geography

descriptive, 234-299

floristic, 15

historical development, 15, 211-
212, 234

of North America, 234-299
Plant indicators, 350-359

agricultural, 352-355

forest site, 357-359

nature and use, 350-352

range and pasture, 355-357
Plant nutrients, 179-182

effect on distribution, 180
Plant sociology, 13, 55-74, 233
Plant succession, 211-233
Platanus occidentalism 255
Playas, 286
Pneumatophores, 176

382

THE STUDY OF PLANT COMMUNITIES

Poa, 268

pratejisis, 292, 293
Pocosins, 257
Podsolization, 155-156
Pollen, wind-borne, 107-108

amounts, 107-108

characteristics, 108

distances, 107-108
Pollen analysis, 304-307

correlation with climate, 306-307

methods, 305-306

peat sampler, 304

pollen diagrams, 305, 306, 307

theory, 304-305
Pollination

animals, 198

devices, 190

wind, 107-108
Polyclimax versus monoclimax, 226-

229
Polygonella polygama, 257
Polystichum spp., 278
Population balance, 207-209
Populus acuminata, 266

angustifolia, 266

sargentii, 266

tremuloides, 243, 244, 262, 266
mycorhiza, 194
Porcupine, damage, 202
Postclimax and preclimax

in altitudinal zonation, 227-228

in latitudinal zonation, 227-228

relicts, 228
Postglacial vegetation, 301-314

progressive changes, 305, 306, 307

reconstruction of (see Pollen an-
alvsis)
Prairie "peninsula," 293-294
Precipitation

and base exchange, 181

and runoff, 93-94

average annual, for U.S., 96

causes, 88-89

effectiveness, 93, 163

evaporation ratio, 96, 97

forms of, 89-90

interception by vegetation, 93-94

measurement, 93-95

seasonal distribution, 93

seasonal variation, 96, 97

source of, 88

records, 95-97

polygonal diagrams, 95, 96
seasonal, 95-96
Preclimax, 227-229

Predators, destruction of, 208
Preferents (fidelity), 72
Presence, 69-71

diagram, 71

scale of, 69

tabulation, 70
Primary succession, 213-214

hydrarch, 216-217

xerarch, 218-219-221
Profile diagrams

topographic, 50

vegetational, 54
Prosopis chilensis, 288, 289

juliflora, 289
Primus serotina, 249
Pseudotsuga, 277, 278, 279

mucronata, 194
mycorhiza, 194

taxifolia, 262, 264
Psychotria punctata, 196
Psychrometer, 81
Pulpwood forest, Maine

composition by phytographs, 63
Purshia, 268

tride?itata, 269, 286

Quadrats, 36-51
kinds, 36-43

chart, 36, 37, 38

experimental, 41

list-count, 36

permanent, 36, 40
mapping, 37, 38

by pantograph, 39
marking, 36, 37, 39, 40, 41
photographing, 38, 39
methods

distribution, 49, 50

in stratified vegetation, 48
nested, 48

random versus systematic, 49, 50
relation of shape to efficiency, 44
shape, 43-44

size and number, 44-49, 46, 48
spacing, 51
Qualitative sociological characters,

64-68
dispersion, 64
periodicity, 65
stratification, 65
sociability, 64
vitality, 64
Quantitative sociological characters
abundance, 56-57

INDEX

383

Quantitative sociological characters
—Continued

cover and space, 61-62

density, 57

frequency, 57, 59-61
Quantitative studies

of climax, 231

of succession, 232-233

phytographs, 231
Quercas alba, 245, 248, 253

agrifolia, 140, 281

borealis, 249, 254

catesbaei, 137, 255, 256

vertical leaf position, 137-138

chrysolepis, 275, 281

cinerea, 255, 256

coccinea, 252

douglasii, 275

dumosa, 275, 282

durata, 140

emoryi, 269

fendleri, 269

gambellii, 269

gunnisoni, 269

imbricaria, 254

lyrata, 253

macrocarpa, 252, 254, 269

margaretta, 256

marilandica, 254, 256

montana, 252

phellos, 253

prinus, 253

stellata, 254

undulata, 269

velutina, 254

virginiana, 28, 258, 259

wislizeni, 275, 281

Rabbit damage, 202
Radiant energv, 116-143

light, 129-143

source for earth, 116

temperature, 118-128

visible spectrum, 116
Radiometer, 131-132
Rainfall (see Precipitation)
Rain gauge, 93-95
Range depletion, 322, 323

indicators, 355-357

management, 321-326
ecological principles, 325
ecological studies, 323-324
indicators, 355-357

objectives, 321

results, 322, 323, 324

recovery, 42, 322, 324
Reaction of organisms, 212-213
Relict method, 310-314
Relicts, 310, 311,312,313

ecological usefulness, 311-314

factors in survival, 312-313

interglacial, 302-303

postclimax, 311, 313
Reproduction and temperature, 128
Reseeding range, 324
Respiration and temperature, 128
Rhamnus, 27 A

californica, 275
Rhododendron, 19

californicimij 278

lapponicwn, 239
Rhus, 220

copallina, 219

triloba, 269
Ribes viscosissimum, 272
Rock succession, 218, 219-221

mat formation, 218-220
Rocky Mountain Forest complex,
259-269

Black Hills, 269

climaxes
Douglas fir, 263-265
Engelman spruce— subalpine fir,

261-263
oak-mountain mahogany, 268-269
pinon-juniper, 267-268
ponderosa pine, 265, 266-267

extent, 259-260

zones and climaxes listed, 260
Root distribution

aeration, 174

mapping, 147
Rumex pulcher, 199
Runoff

and frozen soil, 126

on forested and bare land, 94

Sagebrush, 126, 267-268
Sagebrush formation, 284-287

extent, conditions, 284-285

vegetation, 285-287
Salic ornia spp., 286
Saline soils

bordering oceans, 184

in deserts, 184-185

physiological drought, 184

vegetation, 286

water absorption, 184

384

THE STUDY OF PLANT COMMUNITIES

Salt spray, effects, 102, 103

distribution of vegetation, 51, 52
Salt tolerance, 184-185

zonation, 185
Sample plots, 35
Sampling, ecological, 35-54
efficiency, 45-47
random versus systematic, 49, 50
Sand dunes, 111-115, 149-150
as plant habitats, 150
blowouts, 150
moisture conditions, 171
stabilization, 111, 113, 114, 115
Sandhills, Nebraska, 296
Saprophytes, 26-27
Indian pipe, 27
squaw root, 27
Sarcobatus vermiculatus, 185, 286
Savannah, 253, 254
and fire, 254, 256
Sclerophyll, anatomical characteris-
tics, 140
Seasonal
aspect, 65, 67
dominance, 65
Secondary succession, 214-215, 221-
225
after cultivation, 221-222
after fire, 215, 254
old fields, 221-222
rate, 215
Selaginella acanthonota, 257
Selectives (fidelity), 72
Self pruning, 137
Sequoia gigantea, 273, 274

sempervirens, 278, 279
Shade

and seed production, 140
leaves
characteristics, 137-139
where found, 137
plants, 132
tolerance

and photosvnthetic efficiency,

134
practical considerations, 134
relation to succession, 134
water versus light, 134, 135
Sierra Nevada forest complex, 269-
276
east slope, 275-276
foothills (woodland) zone, 274,

275-276
montane zone, 274

range, climate, altitudes, 269-271
subalpine zone, 270, 271-274
Silvics and ecology, 317-321
artificial forest types, 317-319-320
continuous production, 320
plant sucession, 317, 320
pure and mixed stands, 317
virgin and climax forests, 321
Short grass plains, 295, 296-297, 312
Sisymbrium altissimum, 206
Sleet, 90
Slope
exposure, 124, 125, 133
relation to light and temperature,
133
Smilax laurifolia, 257
Snow
as an insulator, 127
effects on water supply, 68, 69
in subalpine areas, 91-92
measurement of fall, 94-95
Sierra Nevada, 92, 271
source of soil moisture, 90-91
water content, 95
Sociability, gregariousness, disper-
sion, 64
Sociological analysis
objectives and procedure, 74
summary of concepts, 73
Sociological data, application, 56
Soil, 144-161
acidity, 178-179

and soil organisms, 178
decreases with depth, 1 78
H ion concentration, 178
relation to precipitation, 1 78
acid or alkaline reaction, 145-146
aeration

poorer with depth, 1 74

root growth and distribution,

174-175
type of vegetation, 175
air analysis, 177
air capacity, 175, 177
aggregation and alkalinity, 183
alkali, 184
alkalinity, 183
animals, 200-201
atmosphere, 76-77, 148, 174-177
and root growth, 77
and soil organisms, 77
changes with depth, 174
determination of volume and

composition, 177
relation to water content, 177

INDEX

385

Soil, atmosphere— Continued

respiration, 174

winter and summer, 174
base exchange, 179-182
capacity, 180
classification

by mature profiles, 155

climatic, 155, 157-158, 160

zonal, 157, 160
conservation, 338-341

strip cropping and terraces, 340
defined, 144
development, 147

and vegetation, 159-161

biological activity, 146

moisture, 159

temperature relationships, 159
flora

organisms free in soil, 197-198

symbiotic fungi and bacteria,
194-196
formation, 144-147

agents, 144

carbonation, 145

oxidation and hydration, 145

processes, 144
horizons, 146, 147

and root distribution, 147
litter, 153

major components, 148
organic content, and microorgan-
isms, 146, 153-154
origin, 153-154

fermentation (F) layer, 154

humus (H) layer, 154

litter (L) layer, 153
organisms

and acidity, 178

growth inhibiting substances, 182
origin

cumulose soils, 148-149

residual soils, 148
sedentary soils, 148

transported soils, 152
plant relationships, 148
point cones, 173-174
porosity, 177
profile, 146, 147-148

and weathering, 147

development of, 147

processes of development
calcification, 156-157
laterization, 156-157
podsolization, 155-156
salinity, 184-185

sampler, 167
samples, in place, 167
solution, 171
source of nutrients, 153
structure, 153

aggregation, 153

determines air capacity, 175

effect of colloids, 153

shrinkage on drying, 153

single grain, 153
temperature, 122, 123

daily and seasonal lag, 122

extremes at surface, 119

forest litter, 127

germination, 126

modified by cover, 126

relation to atmosphere, 122

seedling survival, 126

water content, 171

water relations, 126, 171
texture, 152-153

a basis of classification, 1 52

and naming of soils, 153

and shrinkage, 153

mechanical analysis, 152
total pore volume, 177
transported

alluvial, 149-150

by ice, 151

colluvial, 150

colluvial cones, 150

dunes, 111-115, 149-150

glacial moraine, 151

loess, 112-113, 149
types and climate, 157-159
types, climatic, 155, 157
variations, local, 148-154

in origin, 148-154

structure, 148

texture, 148
variations, regional, 154-161

relation to climate, 154-155
water, 161-174

and competition, 30

and temperature, 126

availability to plants, 170-171

classification, 161-162

constants, 166-171

capillary capacity, 168
capillary potential, 164
field capacity, 164, 168
hygroscopic coefficient, 167
maximum water holding

capacity, 168
moisture equivalent, 169

386

THE STUDY OF PLANT COMMUNITIES

Soil, water constants— Continued

permanent wilting percentage,

169-170
readily available water, 1 70
wilting coefficient, 169
evaporation loss, 165-166
infiltration, 163
loss by transpiration, 166
measurements, 172-174
content, 172

electrometric methods, 173
expression of, 172
forces with which held, 172
of variations, 172
physical forces, 173
sampling and weighing, 1 72
soil point cones, 173
tensiometers, 173
weight versus volume basis,
172
movement, 163-165
origin, 162-163
weathering, 144
well, 146, 147-148
Solstices, 116-117
Sonoran desert, 287, 288
Sorbus americana, 63
Sorghastrimi nutans, 292, 293
Space (occupied), 61-62
clipping and weighing, 62
estimation of volume, 62
relation to basal area, 62
Spartina pectinata, 292
Species : area curve, 45, 47
Spectrum, 118
Sporobolus cryptandrus, 293, 295

heterolepis, 292, 293
Stand, 21,22,23, 45
Stipa, 268

eomata, 293, 295, 297
leucotricba, 293
pulchra, 297
spartea, 292, 293, 295
Stipulicida setae ea, 257
Stomatal activity, 135-136
response to drought, 136
response to light, 135
Strangers (fidelity), 72
Stratification, 22, 25, 66
and dependent species, 23
and dominance, 23
and layer communities, 25
and sampling, 65
causes, 22
diagrams, 65

dependence, 26

light, 132

sampling methods, 48

studied with bisects, 54

subordinate species, 23

synusia, 25
Stratification-cover diagrams, 65, 66
Stream margins, 330
Street lights

photoperiod, 142

winter killing of trees, 142
Subalpine forest

Rockies, 261-263

Sierra Nevada, 270, 271-274
Subclimax, 226
Subordinate species, 23
Succession

after fire, 242

causes, 212-214

concept, 212

historical background, 211-212

in old fields, 221-222

kinds, 214-216

primary and secondary, 213-214

quantitative studies, 231, 232, 233

rate, 221-223

stabilization and climax, 223-224
Successional diagram, 213
Sun leaves

characteristics, 137-139

where found, 137
Sunspot activity

climate and tree growth, 310
Swamp

forests, 258-259

succession, 217

vegetation, 217
Symbiosis, 193-196

mycorhiza, 194-195

nodules, 195-196
Sy?nphoricarpos spp., 269

rotundifolius, 272
Synecology, defined, 17
Synthetic characteristics, 69-74

coefficient of community, 74

Constance, 71

fidelity, 72

presence, 69, 70, 71
Synusia, 25

forest floor, 23-24

Taiga, 240

Tall grass prairie, 291-292-294

Talus, 150-151

INDEX

387

Taraxacum, 199
Taxodium distichum, 176, 257
buttresses, 176
knees, 176
Temperature, 118-128

and atmospheric pressure, 97
and cover, 125-127
and germination, 127
and growth, 128
and reproduction, 128
and water relations, 128
and weathering, 144
extremes, 125
forest versus open, 125-126
general plant relationships, 118-119
hardwood forest, 127
means, 121-122
widest fluctuations, 144
Temperature adjustments
alpine and arctic plants, 119
hardening, 119
seasonal, 119
Temperature and physiological proc-
esses, 127-128
Temperature measurement
instruments, 119, 120, 121
maximum and minimum ther-
mometers, 120, 121
procedures, soil and air, 119
thermograph, 119-120
thermometers, 119, 120, 121
Temperature ranges
for germination, 127
for species, 118, 119
photosynthesis, 128
water content of protoplasm, 118
Temperature records, 121-122
annual mean, 122
computing means, 121-122
continuous, 119-120, 121
mean, 121

relation of mean to duration, 121
value of extremes, 122
Temperature tolerances
conifers, 118
desert plants, 118
hardening, 119
optimum, maximum, minimum,

127
seeds, and spores, 118
Temperature variations

altitudinal range of species, 124
clouds or fog, 125
differ with insolation, 122
exposure, 124-125, 133

follow insolation, 122

lag, 122

large bodies of water, 122-123

slope, north and south, 124, 126

133
soil, 122
soil surface, 119
valleys and ridges, 98, 124
Temperature zones, 15, 122
in mountains, 124
latitudinal, 122, 124
Merriam's, 15, 324
disrupted
by cold air drainage, 124
by lakes, 123
by mountains, 124
by slope and exposure, 124
Tensiometer, 173
Thermograph, 119, 120

soil-air, 120
Thermometers

maximum and minimum, 120, 121
standard, 119
Thinning

artificial (of forest stands), 24
natural, 24
Thuja occidentalism 242, 251

plicata, 277, 279
Tillandsia usneioides, 28, 193
Tilia a?nerica?2a, and spp., 247, 248.,

249, 250
Topography as a factor, 185-187
effects are indirect, 185-186
local and regional, 186-187
Tortula pagorimi, 193
Toxicity
aeration, 182

carbon dioxide in soil, 174
experimental evidence, 182
high acidity, 179
soil solution, 171
Transects
data, 52

early land surveys, 53
mapping, 51
sizes, 52

transition zones, 52
uses, 51

variations in methods, 53
zonation, 52
Transitions, 30, 224

forest and grassland, 29, 31
Transpiration, measurement of, 82-83
cobalt chloride method, 83
phytometers, 82, 83

388

THE STUDY OF PLANT COMMUNITIES

Tree-ring studies, 308-310
Trenched plots and shade tolerance,

134, 135
Tropical formations, 298-299

factors, 298

growth forms, 298

listed, 299

rain forest, 298
Tsuga canadensis, 248, 249, 250

heterophylla, 277, 279

mertensiana, 70, 261, 272, 276, 279
Tundra formation, 236-240

alpine tundra, 236, 239

arctic tundra, 238

Ulmus, 255, 269
a?nericana, 249
Umbellularia calif ornica, 281
Uniola paniculata, 51, 52

Vaccinium spp., 242, 278
Vapor pressure deficit, 78-80

application, 79

nomogram, 83

relation to relative humidity, 80

significance, 79
Vegetational analysis

basis for other work, 34

methods, 33-54

objectives, 55-74

quantitative data a necessity, 34

sampling, 35
Vegetational changes, historical, 301-
314

climatic parallels, 301-302

modern evidences, 301

relations to glaciation, 301
Vegetation girdles

aquatic, 216

rock succession, 219-220
Vegetation type

and climate, 29

local variations, 29, 30

variation of species, 30
Vegetation types, North America,

235
Vegetation zones, disrupted

by angle of slope, 124, 133

by exposure, 124, 125
Vegetation zones, Rockies, 260

extent, 260

factors involved, 259

foothills (woodland), 267-269

montane, 263-267

subalpine, 261-263

tundra, 239-240
Viburnum alnifolium, 241

cassinoides, 241
Virgin forest, need for study, 321
Vitality, 64

classes, 65

indicator significance, 65

Warm front (air masses), 87
Water

capillary, 162
gravitational, 161
hygroscopic, 162
of the atmosphere, 77-97
of the soil, 161-174
solvent in soil formation, 145
Water absorption and movement

modified by temperature, 128
Water balance and temperature, 128
Water holding capacity, 168
■ Water conservation, 341 -342-343-344
pollution, 344
trends, 344
Water supply, and snow, 68
Water table

and evaporation from soil, 165-166
hydrarch succession, 219
Weathering
biological, 144
chemical, 144
hydration, 145
oxidation, 145
physical, 144
soil acids, 145
Wildlife conservation, 345-347
ecological problems, 345-346
management, 345
refuges, 346
Wilting coefficient, 169
Wilting percentage, 169-170
Wind, 97-115
and atmospheric pressure, 97-98
anemometers, 99
daily and seasonal variation, 98
effects on plants, 99-105
general pattern, 97-98
Krummholz, 101, 102
measurement, 99
physiological-anatomical effects,

99-100
salt spray, 102-103
Wind and soil, 109-115

INDEX

Wind and soil— Continued

blowouts, 111, 112

buried forest, 112, 114

during droughts, 110, 112

loess, 110, 112-113

sand dunes, 111, 113
Wind, coastal, night and day breezes,

98
Wind, in mountains

cold air drainage, 98

valley breezes, 98
Wind, physical effects

flagform, 106

windthrow, 104, 105
Wind, transportation

of disseminules, 108-112

of pollen, 107-108
Witches brooms, 191-192

Xanthium canadense, 199

Xerarch succession, 213-218-219-221

arctic, 238-239
Xeric habitats and leaf structure, 139
Xeromorphism

in halophytes, 184

transpiration, 184

Yucca 290

brevifolia, 287, 288

Zephyranthes atamasco, 68
Zones of vegetation (see Vegeta-
tion Zones)

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